Configuring IP Unicast Routing

Configuring IP Unicast Routing
CH A P T E R
41
Configuring IP Unicast Routing
This chapter describes how to configure IP Version 4 (IPv4) unicast routing on the Catalyst 3750-X
or 3560-X switch.
Note
Routing is not supported on switches running the LAN base feature set.
Unless otherwise noted, the term switch refers to a Catalyst 3750-X or 3560-X standalone switch and to
a Catalyst 3750-X switch stack. A switch stack operates and appears as a single router to the rest of the
routers in the network. Basic routing functions, including static routing and the Routing Information
Protocol (RIP), are available with both the IP base feature set and the IP services feature set. To use
advanced routing features and other routing protocols, you must have the IP services feature set enabled
on the standalone switch or on the stack master.
Note
If the switch or switch stack is running the IP services feature set, you can also enable IP Version 6
(IPv6) unicast routing and configure interfaces to forward IPv6 traffic in addition to IPv4 traffic. For
information about configuring IPv6 on the switch, see Chapter 42, “Configuring IPv6 Unicast Routing.”
For more detailed IP unicast configuration information, see the Cisco IOS IP Configuration Guide,
Release 12.2. For complete syntax and usage information for the commands used in this chapter, see
these command references:
•
Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2
•
Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2
•
Cisco IOS IP Command Reference, Volume 3 of 3: Multicast, Release 12.2
This chapter consists of these sections:
•
Understanding IP Routing, page 41-2
•
Steps for Configuring Routing, page 41-5
•
Configuring IP Addressing, page 41-6
•
Enabling IP Unicast Routing, page 41-19
•
Configuring RIP, page 41-20
•
Configuring OSPF, page 41-26
•
Configuring EIGRP, page 41-36
•
Configuring BGP, page 41-44
•
Configuring ISO CLNS Routing, page 41-65
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Understanding IP Routing
Note
•
Configuring Multi-VRF CE, page 41-75
•
Configuring Protocol-Independent Features, page 41-90
•
Monitoring and Maintaining the IP Network, page 41-105
When configuring routing parameters on the switch and to allocate system resources to maximize the
number of unicast routes allowed, you can use the sdm prefer routing global configuration command
to set the Switch Database Management (sdm) feature to the routing template. For more information on
the SDM templates, see Chapter 8, “Configuring SDM Templates” or see the sdm prefer command in
the command reference for this release.
Understanding IP Routing
In some network environments, VLANs are associated with individual networks or subnetworks. In an
IP network, each subnetwork is mapped to an individual VLAN. Configuring VLANs helps control the
size of the broadcast domain and keeps local traffic local. However, network devices in different VLANs
cannot communicate with one another without a Layer 3 device (router) to route traffic between the
VLAN, referred to as inter-VLAN routing. You configure one or more routers to route traffic to the
appropriate destination VLAN.
Figure 41-1 shows a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20. The
router has an interface in each VLAN.
Routing Topology Example
VLAN 10
A
Host
VLAN 20
Switch A
Switch B
C
Host
B
Host
ISL Trunks
18071
Figure 41-1
When Host A in VLAN 10 needs to communicate with Host B in VLAN 10, it sends a packet addressed
to that host. Switch A forwards the packet directly to Host B, without sending it to the router.
When Host A sends a packet to Host C in VLAN 20, Switch A forwards the packet to the router, which
receives the traffic on the VLAN 10 interface. The router checks the routing table, finds the correct
outgoing interface, and forwards the packet on the VLAN 20 interface to Switch B. Switch B receives
the packet and forwards it to Host C.
This section contains information on these routing topics:
•
Types of Routing, page 41-3
•
IP Routing and Switch Stacks, page 41-3
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Types of Routing
Routers and Layer 3 switches can route packets in three different ways:
•
By using default routing
•
By using preprogrammed static routes for the traffic
•
By dynamically calculating routes by using a routing protocol
Default routing refers to sending traffic with a destination unknown to the router to a default outlet
or destination.
Static unicast routing forwards packets from predetermined ports through a single path into and out of a
network. Static routing is secure and uses little bandwidth, but does not automatically respond to changes
in the network, such as link failures, and therefore, might result in unreachable destinations. As networks
grow, static routing becomes a labor-intensive liability.
Dynamic routing protocols are used by routers to dynamically calculate the best route for forwarding
traffic. There are two types of dynamic routing protocols:
•
Routers using distance-vector protocols maintain routing tables with distance values of networked
resources, and periodically pass these tables to their neighbors. Distance-vector protocols use one
or a series of metrics for calculating the best routes. These protocols are easy to configure and use.
•
Routers using link-state protocols maintain a complex database of network topology, based on the
exchange of link-state advertisements (LSAs) between routers. LSAs are triggered by an event in
the network, which speeds up the convergence time or time required to respond to these changes.
Link-state protocols respond quickly to topology changes, but require greater bandwidth and more
resources than distance-vector protocols.
Distance-vector protocols supported by the switch are Routing Information Protocol (RIP), which uses
a single distance metric (cost) to determine the best path and Border Gateway Protocol (BGP), which
adds a path vector mechanism. The switch also supports the Open Shortest Path First (OSPF) link-state
protocol and Enhanced IGRP (EIGRP), which adds some link-state routing features to traditional
Interior Gateway Routing Protocol (IGRP) to improve efficiency.
Note
On a switch or switch stack, the supported protocols are determined by the software running on the
switch or stack master. If the switch or stack master is running the IP base feature set, only default
routing, static routing and RIP are supported. All other routing protocols require the IP services feature
set.
IP Routing and Switch Stacks
A switch stack appears to the network as a single router, regardless of which switch in the stack is
connected to a routing peer. For additional information about switch stack operation, see Chapter 5,
“Managing Switch Stacks.”
The stack master performs these functions:
•
It initializes and configures the routing protocols.
•
It sends routing protocol messages and updates to other routers.
•
It processes routing protocol messages and updates received from peer routers.
•
It generates, maintains, and distributes the distributed Cisco Express Forwarding (dCEF) database
to all stack members. The routes are programmed on all switches in the stack bases on this database.
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Understanding IP Routing
•
The MAC address of the stack master is used as the router MAC address for the whole stack, and all
outside devices use this address to send IP packets to the stack.
•
All IP packets that require software forwarding or processing go through the CPU of the stack
master.
Stack members perform these functions:
•
They act as routing standby switches, ready to take over in case they are elected as the new stack
master if the stack master fails.
•
They program the routes into hardware. The routes programmed by the stack members are the same
that are downloaded by the stack master as part of the dCEF database.
If a stack master fails, the stack detects that the stack master is down and elects one of the stack members
to be the new stack master. During this period, except for a momentary interruption, the hardware
continues to forward packets with no active protocols.
However, even though the switch stack maintains the hardware identification after a failure, the routing
protocols on the router neighbors might flap during the brief interruption before the stack master restarts.
Routing protocols such as OSPF and EIGRP need to recognize neighbor transitions. The router uses two
levels of nonstop forwarding (NSF) to detect a switchover, to continue forwarding network traffic, and
to recover route information from peer devices:
•
NSF-aware routers tolerate neighboring router failures. After the neighbor router restarts, an
NSF-aware router supplies information about its state and route adjacencies on request.
•
NSF-capable routers support NSF. When they detect a stack master change, they rebuild routing
information from NSF-aware or NSF-capable neighbors and do not wait for a restart.
The switch stack supports NSF-capable routing for OSPF and EIGRP. For more information, see the
“OSPF NSF Capability” section on page 41-29 and the “EIGRP NSF Capability” section on page 41-39.
Upon election, the new stack master performs these functions:
•
It starts generating, receiving, and processing routing updates.
•
It builds routing tables, generates the CEF database, and distributes it to stack members.
•
It uses its MAC address as the router MAC address. To notify its network peers of the new MAC
address, it periodically (every few seconds for 5 minutes) sends a gratuitous ARP reply with the new
router MAC address.
Note
•
Note
If you configure the persistent MAC address feature on the stack and the stack master
changes, the stack MAC address does not change for the configured time period. If the
previous stack master rejoins the stack as a member switch during that time period, the stack
MAC address remains the MAC address of the previous stack master. See the “Enabling
Persistent MAC Address” section on page 5-20.
It attempts to determine the reachability of every proxy ARP entry by sending an ARP request to the
proxy ARP IP address and receiving an ARP reply. For each reachable proxy ARP IP address, it
generates a gratuitous ARP reply with the new router MAC address. This process is repeated for
5 minutes after a new stack master election.
When a stack master is running the IP services feature set, the stack can to run all supported protocols,
including Open Shortest Path First (OSPF), Enhanced IGRP (EIGRP), and Border Gateway Protocol
(BGP). If the stack master fails and the new elected stack master is running the IP base feature set, these
protocols will no longer run in the stack.
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Steps for Configuring Routing
Caution
Partitioning of the switch stack into two or more stacks might lead to undesirable behavior in the
network.
Steps for Configuring Routing
By default, IP routing is disabled on the switch, and you must enable it before routing can take place.
For detailed IP routing configuration information, see the Cisco IOS IP Configuration Guide,
Release 12.2
In the following procedures, the specified interface must be one of these Layer 3 interfaces:
Note
•
A routed port: a physical port configured as a Layer 3 port by using the no switchport interface
configuration command.
•
A switch virtual interface (SVI): a VLAN interface created by using the interface vlan vlan_id
global configuration command and by default a Layer 3 interface.
•
An EtherChannel port channel in Layer 3 mode: a port-channel logical interface created by using
the interface port-channel port-channel-number global configuration command and binding the
Ethernet interface into the channel group. For more information, see the “Configuring Layer 3
EtherChannels” section on page 39-15.
The switch does not support tunnel interfaces for unicast routed traffic.
All Layer 3 interfaces on which routing will occur must have IP addresses assigned to them. See the
“Assigning IP Addresses to Network Interfaces” section on page 41-7.
Note
A Layer 3 switch can have an IP address assigned to each routed port and SVI. The number of routed
ports and SVIs that you can configure is not limited by software. However, the interrelationship between
this number and the number and volume of features being implemented might have an impact on CPU
utilization because of hardware limitations. To optimize system memory for routing, use the sdm prefer
routing global configuration command.
Configuring routing consists of several main procedures:
•
To support VLAN interfaces, create and configure VLANs on the switch or switch stack, and assign
VLAN membership to Layer 2 interfaces. For more information, see Chapter 14, “Configuring
VLANs.”
•
Configure Layer 3 interfaces.
•
Enable IP routing on the switch.
•
Assign IP addresses to the Layer 3 interfaces.
•
Enable selected routing protocols on the switch.
•
Configure routing protocol parameters (optional).
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Configuring IP Addressing
Configuring IP Addressing
A required task for configuring IP routing is to assign IP addresses to Layer 3 network interfaces to
enable the interfaces and allow communication with the hosts on those interfaces that use IP. These
sections describe how to configure various IP addressing features. Assigning IP addresses to the
interface is required; the other procedures are optional.
•
Default Addressing Configuration, page 41-6
•
Assigning IP Addresses to Network Interfaces, page 41-7
•
Configuring Address Resolution Methods, page 41-9
•
Routing Assistance When IP Routing is Disabled, page 41-12
•
Configuring Broadcast Packet Handling, page 41-14
•
Monitoring and Maintaining IP Addressing, page 41-18
Default Addressing Configuration
Table 41-1
Default Addressing Configuration
Feature
Default Setting
IP address
None defined.
ARP
No permanent entries in the Address Resolution Protocol (ARP) cache.
Encapsulation: Standard Ethernet-style ARP.
Timeout: 14400 seconds (4 hours).
IP broadcast address
255.255.255.255 (all ones).
IP classless routing
Enabled.
IP default gateway
Disabled.
IP directed broadcast
Disabled (all IP directed broadcasts are dropped).
IP domain
Domain list: No domain names defined.
Domain lookup: Enabled.
Domain name: Enabled.
IP forward-protocol
If a helper address is defined or User Datagram Protocol (UDP) flooding is configured, UDP
forwarding is enabled on default ports.
Any-local-broadcast: Disabled.
Spanning Tree Protocol (STP): Disabled.
Turbo-flood: Disabled.
IP helper address
Disabled.
IP host
Disabled.
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Table 41-1
Default Addressing Configuration (continued)
Feature
Default Setting
IRDP
Disabled.
Defaults when enabled:
•
Broadcast IRDP advertisements.
•
Maximum interval between advertisements: 600 seconds.
•
Minimum interval between advertisements: 0.75 times max interval
•
Preference: 0.
IP proxy ARP
Enabled.
IP routing
Disabled.
IP subnet-zero
Disabled.
Assigning IP Addresses to Network Interfaces
An IP address identifies a location to which IP packets can be sent. Some IP addresses are reserved for
special uses and cannot be used for host, subnet, or network addresses. RFC 1166, “Internet Numbers,”
contains the official description of IP addresses.
An interface can have one primary IP address. A mask identifies the bits that denote the network number
in an IP address. When you use the mask to subnet a network, the mask is referred to as a subnet mask.
To receive an assigned network number, contact your Internet service provider.
Beginning in privileged EXEC mode, follow these steps to assign an IP address and a network mask to
a Layer 3 interface:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the Layer 3
interface to configure.
Step 3
no switchport
Remove the interface from Layer 2 configuration mode (if it is a
physical interface).
Step 4
ip address ip-address subnet-mask
Configure the IP address and IP subnet mask.
Step 5
no shutdown
Enable the interface.
Step 6
end
Return to privileged EXEC mode.
Step 7
show interfaces [interface-id]
show ip interface [interface-id]
show running-config interface [interface-id]
Verify your entries.
Step 8
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use of Subnet Zero
Subnetting with a subnet address of zero is strongly discouraged because of the problems that can arise
if a network and a subnet have the same addresses. For example, if network 131.108.0.0 is subnetted as
255.255.255.0, subnet zero would be written as 131.108.0.0, which is the same as the network address.
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You can use the all ones subnet (131.108.255.0) and even though it is discouraged, you can enable the
use of subnet zero if you need the entire subnet space for your IP address.
Beginning in privileged EXEC mode, follow these steps to enable subnet zero:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip subnet-zero
Enable the use of subnet zero for interface addresses and routing updates.
Step 3
end
Return to privileged EXEC mode.
Step 4
show running-config
Verify your entry.
Step 5
copy running-config startup-config
(Optional) Save your entry in the configuration file.
Use the no ip subnet-zero global configuration command to restore the default and disable the use of
subnet zero.
Classless Routing
By default, classless routing behavior is enabled on the switch when it is configured to route. With
classless routing, if a router receives packets for a subnet of a network with no default route, the router
forwards the packet to the best supernet route. A supernet consists of contiguous blocks of Class C
address spaces used to simulate a single, larger address space and is designed to relieve the pressure on
the rapidly depleting Class B address space.
In Figure 41-2, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of
discarding the packet, the router forwards it to the best supernet route. If you disable classless routing
and a router receives packets destined for a subnet of a network with no network default route, the router
discards the packet.
Figure 41-2
IP Classless Routing
128.0.0.0/8
128.20.4.1
128.20.0.0
128.20.1.0
IP classless
128.20.3.0
128.20.4.1
Host
45749
128.20.2.0
In Figure 41-3, the router in network 128.20.0.0 is connected to subnets 128.20.1.0, 128.20.2.0, and
128.20.3.0. If the host sends a packet to 120.20.4.1, because there is no network default route, the router
discards the packet.
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Figure 41-3
No IP Classless Routing
128.0.0.0/8
128.20.4.1
128.20.0.0
Bit bucket
128.20.1.0
128.20.3.0
128.20.4.1
Host
45748
128.20.2.0
To prevent the switch from forwarding packets destined for unrecognized subnets to the best supernet
route possible, you can disable classless routing behavior.
Beginning in privileged EXEC mode, follow these steps to disable classless routing:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
no ip classless
Disable classless routing behavior.
Step 3
end
Return to privileged EXEC mode.
Step 4
show running-config
Verify your entry.
Step 5
copy running-config startup-config
(Optional) Save your entry in the configuration file.
To restore the default and have the switch forward packets destined for a subnet of a network with no
network default route to the best supernet route possible, use the ip classless global configuration
command.
Configuring Address Resolution Methods
You can control interface-specific handling of IP by using address resolution. A device using IP can have
both a local address or MAC address, which uniquely defines the device on its local segment or LAN,
and a network address, which identifies the network to which the device belongs.
Note
In a switch stack, network communication uses a single MAC address and the IP address of the stack.
The local address or MAC address is known as a data link address because it is contained in the data link
layer (Layer 2) section of the packet header and is read by data link (Layer 2) devices. To communicate
with a device on Ethernet, the software must learn the MAC address of the device. The process of
learning the MAC address from an IP address is called address resolution. The process of learning the
IP address from the MAC address is called reverse address resolution.
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The switch can use these forms of address resolution:
•
Address Resolution Protocol (ARP) is used to associate IP address with MAC addresses. Taking an
IP address as input, ARP learns the associated MAC address and then stores the IP address/MAC
address association in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a
link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP requests or
replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access Protocol
(SNAP).
•
Proxy ARP helps hosts with no routing tables learn the MAC addresses of hosts on other networks
or subnets. If the switch (router) receives an ARP request for a host that is not on the same interface
as the ARP request sender, and if the router has all of its routes to the host through other interfaces,
it generates a proxy ARP packet giving its own local data link address. The host that sent the ARP
request then sends its packets to the router, which forwards them to the intended host.
The switch also uses the Reverse Address Resolution Protocol (RARP), which functions the same as
ARP does, except that the RARP packets request an IP address instead of a local MAC address. Using
RARP requires a RARP server on the same network segment as the router interface. Use the ip
rarp-server address interface configuration command to identify the server.
For more information on RARP, see the Cisco IOS Configuration Fundamentals Configuration Guide,
Release 12.2.
You can perform these tasks to configure address resolution:
•
Define a Static ARP Cache, page 41-10
•
Set ARP Encapsulation, page 41-11
•
Enable Proxy ARP, page 41-12
Define a Static ARP Cache
ARP and other address resolution protocols provide dynamic mapping between IP addresses and MAC
addresses. Because most hosts support dynamic address resolution, you usually do not need to specify
static ARP cache entries. If you must define a static ARP cache entry, you can do so globally, which
installs a permanent entry in the ARP cache that the switch uses to translate IP addresses into MAC
addresses. Optionally, you can also specify that the switch respond to ARP requests as if it were the
owner of the specified IP address. If you do not want the ARP entry to be permanent, you can specify a
timeout period for the ARP entry.
Beginning in privileged EXEC mode, follow these steps to provide static mapping between IP addresses
and MAC addresses:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
arp ip-address hardware-address type
Globally associate an IP address with a MAC (hardware) address
in the ARP cache, and specify encapsulation type as one of
these:
•
arpa—ARP encapsulation for Ethernet interfaces
•
snap—Subnetwork Address Protocol encapsulation for
Token Ring and FDDI interfaces
•
sap—HP’s ARP type
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Command
Purpose
Step 3
arp ip-address hardware-address type [alias]
(Optional) Specify that the switch respond to ARP requests as if
it were the owner of the specified IP address.
Step 4
interface interface-id
Enter interface configuration mode, and specify the interface to
configure.
Step 5
arp timeout seconds
(Optional) Set the length of time an ARP cache entry will stay in
the cache. The default is 14400 seconds (4 hours). The range is
0 to 2147483 seconds.
Step 6
end
Return to privileged EXEC mode.
Step 7
show interfaces [interface-id]
Verify the type of ARP and the timeout value used on all
interfaces or a specific interface.
Step 8
show arp
View the contents of the ARP cache.
or
show ip arp
Step 9
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To remove an entry from the ARP cache, use the no arp ip-address hardware-address type global
configuration command. To remove all nonstatic entries from the ARP cache, use the clear arp-cache
privileged EXEC command.
Set ARP Encapsulation
By default, Ethernet ARP encapsulation (represented by the arpa keyword) is enabled on an IP interface.
You can change the encapsulation methods to SNAP if required by your network.
Beginning in privileged EXEC mode, follow these steps to specify the ARP encapsulation type:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the Layer 3
interface to configure.
Step 3
arp {arpa | snap}
Specify the ARP encapsulation method:
•
arpa—Address Resolution Protocol
•
snap—Subnetwork Address Protocol
Step 4
end
Return to privileged EXEC mode.
Step 5
show interfaces [interface-id]
Verify ARP encapsulation configuration on all interfaces or
the specified interface.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To disable an encapsulation type, use the no arp arpa or no arp snap interface configuration command.
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Enable Proxy ARP
By default, the switch uses proxy ARP to help hosts learn MAC addresses of hosts on other networks or
subnets.
Beginning in privileged EXEC mode, follow these steps to enable proxy ARP if it has been disabled:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the Layer 3
interface to configure.
Step 3
ip proxy-arp
Enable proxy ARP on the interface.
Step 4
end
Return to privileged EXEC mode.
Step 5
show ip interface [interface-id]
Verify the configuration on the interface or all interfaces.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To disable proxy ARP on the interface, use the no ip proxy-arp interface configuration command.
Routing Assistance When IP Routing is Disabled
These mechanisms allow the switch to learn about routes to other networks when it does not have IP
routing enabled:
•
Proxy ARP, page 41-12
•
Default Gateway, page 41-12
•
ICMP Router Discovery Protocol (IRDP), page 41-13
Proxy ARP
Proxy ARP, the most common method for learning about other routes, enables an Ethernet host with no
routing information to communicate with hosts on other networks or subnets. The host assumes that all
hosts are on the same local Ethernet and that they can use ARP to learn their MAC addresses. If a switch
receives an ARP request for a host that is not on the same network as the sender, the switch evaluates
whether it has the best route to that host. If it does, it sends an ARP reply packet with its own Ethernet
MAC address, and the host that sent the request sends the packet to the switch, which forwards it to the
intended host. Proxy ARP treats all networks as if they are local and performs ARP requests for every
IP address.
Proxy ARP is enabled by default. To enable it after it has been disabled, see the “Enable Proxy ARP”
section on page 41-12. Proxy ARP works as long as other routers support it.
Default Gateway
Another method for locating routes is to define a default router or default gateway. All nonlocal packets
are sent to this router, which either routes them appropriately or sends an IP Control Message Protocol
(ICMP) redirect message back, defining which local router the host should use. The switch caches the
redirect messages and forwards each packet as efficiently as possible. A limitation of this method is that
there is no means of detecting when the default router has gone down or is unavailable.
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Beginning in privileged EXEC mode, follow these steps to define a default gateway (router) when IP
routing is disabled:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip default-gateway ip-address
Set up a default gateway (router).
Step 3
end
Return to privileged EXEC mode.
Step 4
show ip redirects
Display the address of the default gateway router to verify the
setting.
Step 5
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no ip default-gateway global configuration command to disable this function.
ICMP Router Discovery Protocol (IRDP)
Router discovery allows the switch to dynamically learn about routes to other networks using IRDP.
IRDP allows hosts to locate routers. When operating as a client, the switch generates router discovery
packets. When operating as a host, the switch receives router discovery packets. The switch can also
listen to Routing Information Protocol (RIP) routing updates and use this information to infer locations
of routers. The switch does not actually store the routing tables sent by routing devices; it merely keeps
track of which systems are sending the data. The advantage of using IRDP is that it allows each router
to specify both a priority and the time after which a device is assumed to be down if no further packets
are received.
Each device discovered becomes a candidate for the default router, and a new highest-priority router is
selected when a higher priority router is discovered, when the current default router is declared down,
or when a TCP connection is about to time out because of excessive retransmissions.
The only required task for IRDP routing on an interface is to enable IRDP processing on that interface.
When enabled, the default parameters apply. You can optionally change any of these parameters.
Beginning in privileged EXEC mode, follow these steps to enable and configure IRDP on an interface:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the Layer 3 interface to
configure.
Step 3
ip irdp
Enable IRDP processing on the interface.
Step 4
ip irdp multicast
(Optional) Send IRDP advertisements to the multicast address
(224.0.0.1) instead of IP broadcasts.
Note
Step 5
ip irdp holdtime seconds
This command allows for compatibility with Sun Microsystems
Solaris, which requires IRDP packets to be sent out as multicasts.
Many implementations cannot receive these multicasts; ensure
end-host ability before using this command.
(Optional) Set the IRDP period for which advertisements are valid. The
default is three times the maxadvertinterval value. It must be greater
than maxadvertinterval and cannot be greater than 9000 seconds. If you
change the maxadvertinterval value, this value also changes.
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Configuring IP Addressing
Command
Purpose
Step 6
ip irdp maxadvertinterval seconds
(Optional) Set the IRDP maximum interval between advertisements. The
default is 600 seconds.
Step 7
ip irdp minadvertinterval seconds
(Optional) Set the IRDP minimum interval between advertisements. The
default is 0.75 times the maxadvertinterval. If you change the
maxadvertinterval, this value changes to the new default (0.75 of
maxadvertinterval).
Step 8
ip irdp preference number
(Optional) Set a device IRDP preference level. The allowed range is –231
to 231. The default is 0. A higher value increases the router preference
level.
Step 9
ip irdp address address [number]
(Optional) Specify an IRDP address and preference to proxy-advertise.
Step 10
end
Return to privileged EXEC mode.
Step 11
show ip irdp
Verify settings by displaying IRDP values.
Step 12
copy running-config startup-config
(Optional) Save your entries in the configuration file.
If you change the maxadvertinterval value, the holdtime and minadvertinterval values also change,
so it is important to first change the maxadvertinterval value, before manually changing either the
holdtime or minadvertinterval values.
Use the no ip irdp interface configuration command to disable IRDP routing.
Configuring Broadcast Packet Handling
After configuring an IP interface address, you can enable routing and configure one or more routing
protocols, or you can configure the way the switch responds to network broadcasts. A broadcast is a data
packet destined for all hosts on a physical network. The switch supports two kinds of broadcasting:
Note
•
A directed broadcast packet is sent to a specific network or series of networks. A directed broadcast
address includes the network or subnet fields.
•
A flooded broadcast packet is sent to every network.
You can also limit broadcast, unicast, and multicast traffic on Layer 2 interfaces by using the
storm-control interface configuration command to set traffic suppression levels. For more information,
see Chapter 28, “Configuring Port-Based Traffic Control.”
Routers provide some protection from broadcast storms by limiting their extent to the local cable.
Bridges (including intelligent bridges), because they are Layer 2 devices, forward broadcasts to all
network segments, thus propagating broadcast storms. The best solution to the broadcast storm problem
is to use a single broadcast address scheme on a network. In most modern IP implementations, you can
set the address to be used as the broadcast address. Many implementations, including the one in the
switch, support several addressing schemes for forwarding broadcast messages.
Perform the tasks in these sections to enable these schemes:
•
Enabling Directed Broadcast-to-Physical Broadcast Translation, page 41-15
•
Forwarding UDP Broadcast Packets and Protocols, page 41-16
•
Establishing an IP Broadcast Address, page 41-17
•
Flooding IP Broadcasts, page 41-17
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Enabling Directed Broadcast-to-Physical Broadcast Translation
By default, IP directed broadcasts are dropped; they are not forwarded. Dropping IP-directed broadcasts
makes routers less susceptible to denial-of-service attacks.
You can enable forwarding of IP-directed broadcasts on an interface where the broadcast becomes a
physical (MAC-layer) broadcast. Only those protocols configured by using the ip forward-protocol
global configuration command are forwarded.
You can specify an access list to control which broadcasts are forwarded. When an access list is
specified, only those IP packets permitted by the access list are eligible to be translated from directed
broadcasts to physical broadcasts. For more information on access lists, see Chapter 36, “Configuring
Network Security with ACLs.”
Beginning in privileged EXEC mode, follow these steps to enable forwarding of IP-directed broadcasts
on an interface:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the interface to
configure.
Step 3
ip directed-broadcast [access-list-number]
Enable directed broadcast-to-physical broadcast translation on the
interface. You can include an access list to control which broadcasts
are forwarded. When an access list, only IP packets permitted by the
access list can be translated
Note
The ip directed-broadcast interface configuration command
can be configured on a VPN routing/forwarding(VRF)
interface and is VRF aware. Directed broadcast traffic is
routed only within the VRF.
Step 4
exit
Step 5
ip forward-protocol {udp [port] | nd | sdns} Specify which protocols and ports the router forwards when
forwarding broadcast packets.
Return to global configuration mode.
•
udp—Forward UPD datagrams.
port: (Optional) Destination port that controls which UDP
services are forwarded.
•
nd—Forward ND datagrams.
•
sdns—Forward SDNS datagrams
Step 6
end
Return to privileged EXEC mode.
Step 7
show ip interface [interface-id]
Verify the configuration on the interface or all interfaces.
or
show running-config
Step 8
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no ip directed-broadcast interface configuration command to disable translation of directed
broadcast to physical broadcasts. Use the no ip forward-protocol global configuration command to
remove a protocol or port.
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Configuring IP Addressing
Forwarding UDP Broadcast Packets and Protocols
User Datagram Protocol (UDP) is an IP host-to-host layer protocol, as is TCP. UDP provides a
low-overhead, connectionless session between two end systems and does not provide for
acknowledgment of received datagrams. Network hosts occasionally use UDP broadcasts to find
address, configuration, and name information. If such a host is on a network segment that does not
include a server, UDP broadcasts are normally not forwarded. You can remedy this situation by
configuring an interface on a router to forward certain classes of broadcasts to a helper address. You can
use more than one helper address per interface.
You can specify a UDP destination port to control which UDP services are forwarded. You can specify
multiple UDP protocols. You can also specify the Network Disk (ND) protocol, which is used by older
diskless Sun workstations and the network security protocol SDNS.
By default, both UDP and ND forwarding are enabled if a helper address has been defined for an
interface. The description for the ip forward-protocol interface configuration command in the Cisco
IOS IP Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2 lists the ports that
are forwarded by default if you do not specify any UDP ports.
If you do not specify any UDP ports when you configure the forwarding of UDP broadcasts, you are
configuring the router to act as a BOOTP forwarding agent. BOOTP packets carry DHCP information.
Beginning in privileged EXEC mode, follow these steps to enable forwarding UDP broadcast packets on
an interface and specify the destination address:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the Layer 3 interface
to configure.
Step 3
ip helper-address address
Enable forwarding and specify the destination address for forwarding
UDP broadcast packets, including BOOTP.
Step 4
exit
Return to global configuration mode.
Step 5
ip forward-protocol {udp [port] | nd | sdns} Specify which protocols the router forwards when forwarding
broadcast packets.
Step 6
end
Return to privileged EXEC mode.
Step 7
show ip interface [interface-id]
Verify the configuration on the interface or all interfaces.
or
show running-config
Step 8
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no ip helper-address interface configuration command to disable the forwarding of broadcast
packets to specific addresses. Use the no ip forward-protocol global configuration command to remove
a protocol or port.
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Configuring IP Addressing
Establishing an IP Broadcast Address
The most popular IP broadcast address (and the default) is an address consisting of all ones
(255.255.255.255). However, the switch can be configured to generate any form of IP broadcast address.
Beginning in privileged EXEC mode, follow these steps to set the IP broadcast address on an interface:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the interface to
configure.
Step 3
ip broadcast-address ip-address
Enter a broadcast address different from the default, for example
128.1.255.255.
Step 4
end
Return to privileged EXEC mode.
Step 5
show ip interface [interface-id]
Verify the broadcast address on the interface or all interfaces.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To restore the default IP broadcast address, use the no ip broadcast-address interface configuration
command.
Flooding IP Broadcasts
You can allow IP broadcasts to be flooded throughout your internetwork in a controlled fashion by using
the database created by the bridging STP. Using this feature also prevents loops. To support this
capability, bridging must be configured on each interface that is to participate in the flooding. If bridging
is not configured on an interface, it still can receive broadcasts. However, the interface never forwards
broadcasts it receives, and the router never uses that interface to send broadcasts received on a
different interface.
Packets that are forwarded to a single network address using the IP helper-address mechanism can be
flooded. Only one copy of the packet is sent on each network segment.
To be considered for flooding, packets must meet these criteria. (Note that these are the same conditions
used to consider packet forwarding using IP helper addresses.)
•
The packet must be a MAC-level broadcast.
•
The packet must be an IP-level broadcast.
•
The packet must be a TFTP, DNS, Time, NetBIOS, ND, or BOOTP packet, or a UDP specified by
the ip forward-protocol udp global configuration command.
•
The time-to-live (TTL) value of the packet must be at least two.
A flooded UDP datagram is given the destination address specified with the ip broadcast-address
interface configuration command on the output interface. The destination address can be set to any
address. Thus, the destination address might change as the datagram propagates through the network.
The source address is never changed. The TTL value is decremented.
When a flooded UDP datagram is sent out an interface (and the destination address possibly changed),
the datagram is handed to the normal IP output routines and is, therefore, subject to access lists, if they
are present on the output interface.
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Beginning in privileged EXEC mode, follow these steps to use the bridging spanning-tree database to
flood UDP datagrams:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip forward-protocol spanning-tree
Use the bridging spanning-tree database to flood UDP datagrams.
Step 3
end
Return to privileged EXEC mode.
Step 4
show running-config
Verify your entry.
Step 5
copy running-config startup-config
(Optional) Save your entry in the configuration file.
Use the no ip forward-protocol spanning-tree global configuration command to disable the flooding
of IP broadcasts.
In the switch, the majority of packets are forwarded in hardware; most packets do not go through the
switch CPU. For those packets that do go to the CPU, you can speed up spanning tree-based UDP
flooding by a factor of about four to five times by using turbo-flooding. This feature is supported over
Ethernet interfaces configured for ARP encapsulation.
Beginning in privileged EXEC mode, follow these steps to increase spanning-tree-based flooding:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode
Step 2
ip forward-protocol turbo-flood
Use the spanning-tree database to speed up flooding of UDP datagrams.
Step 3
end
Return to privileged EXEC mode.
Step 4
show running-config
Verify your entry.
Step 5
copy running-config startup-config
(Optional) Save your entry in the configuration file.
To disable this feature, use the no ip forward-protocol turbo-flood global configuration command.
Monitoring and Maintaining IP Addressing
When the contents of a particular cache, table, or database have become or are suspected to be invalid,
you can remove all its contents by using the clear privileged EXEC commands. Table 41-2 lists the
commands for clearing contents.
Table 41-2
Commands to Clear Caches, Tables, and Databases
Command
Purpose
clear arp-cache
Clear the IP ARP cache and the fast-switching cache.
clear host {name | *}
Remove one or all entries from the hostname and the address cache.
clear ip route {network [mask] |*}
Remove one or more routes from the IP routing table.
You can display specific statistics, such as the contents of IP routing tables, caches, and databases; the
reachability of nodes; and the routing path that packets are taking through the network. Table 41-3 lists
the privileged EXEC commands for displaying IP statistics.
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Table 41-3
Commands to Display Caches, Tables, and Databases
Command
Purpose
show arp
Display the entries in the ARP table.
show hosts
Display the default domain name, style of lookup service, name server hosts,
and the cached list of hostnames and addresses.
show ip aliases
Display IP addresses mapped to TCP ports (aliases).
show ip arp
Display the IP ARP cache.
show ip interface [interface-id]
Display the IP status of interfaces.
show ip irdp
Display IRDP values.
show ip masks address
Display the masks used for network addresses and the number of subnets
using each mask.
show ip redirects
Display the address of a default gateway.
show ip route [address [mask]] | [protocol]
Display the current state of the routing table.
show ip route summary
Display the current state of the routing table in summary form.
Enabling IP Unicast Routing
By default, the switch is in Layer 2 switching mode and IP routing is disabled. To use the Layer 3
capabilities of the switch, you must enable IP routing.
Beginning in privileged EXEC mode, follow these steps to enable IP routing:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip routing
Enable IP routing.
Step 3
router ip_routing_protocol
Specify an IP routing protocol. This step might include other
commands, such as specifying the networks to route with the
network (RIP) router configuration command. For information on
specific protocols, see sections later in this chapter and to the Cisco
IOS IP Configuration Guide, Release 12.2.
Note
The IP base feature set supports only RIP as a routing
protocol.
Step 4
end
Return to privileged EXEC mode.
Step 5
show running-config
Verify your entries.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no ip routing global configuration command to disable routing.
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Configuring RIP
This example shows how to enable IP routing using RIP as the routing protocol:
Switch# configure terminal
Enter configuration commands, one per line.
Switch(config)# ip routing
Switch(config)# router rip
Switch(config-router)# network 10.0.0.0
Switch(config-router)# end
End with CNTL/Z.
You can now set up parameters for the selected routing protocols as described in these sections:
•
Configuring RIP, page 41-20
•
Configuring OSPF, page 41-26
•
Configuring EIGRP, page 41-36
•
Configuring BGP, page 41-44
•
Configuring Unicast Reverse Path Forwarding, page 41-90
•
Configuring Protocol-Independent Features, page 41-90 (optional)
Configuring RIP
The Routing Information Protocol (RIP) is an interior gateway protocol (IGP) created for use in small,
homogeneous networks. It is a distance-vector routing protocol that uses broadcast User Datagram
Protocol (UDP) data packets to exchange routing information. The protocol is documented in RFC 1058.
You can find detailed information about RIP in IP Routing Fundamentals, published by Cisco Press.
Note
RIP is the only routing protocol supported by the IP base feature set; other routing protocols require the
switch or stack master to be running the IP services feature set.
Using RIP, the switch sends routing information updates (advertisements) every 30 seconds. If a router
does not receive an update from another router for 180 seconds or more, it marks the routes served by
that router as unusable. If there is still no update after 240 seconds, the router removes all routing table
entries for the non-updating router.
RIP uses hop counts to rate the value of different routes. The hop count is the number of routers that can
be traversed in a route. A directly connected network has a hop count of zero; a network with a hop count
of 16 is unreachable. This small range (0 to 15) makes RIP unsuitable for large networks.
If the router has a default network path, RIP advertises a route that links the router to the pseudonetwork
0.0.0.0. The 0.0.0.0 network does not exist; it is treated by RIP as a network to implement the default
routing feature. The switch advertises the default network if a default was learned by RIP or if the router
has a gateway of last resort and RIP is configured with a default metric. RIP sends updates to the
interfaces in specified networks. If an interface’s network is not specified, it is not advertised in any
RIP update.
These sections contain this configuration information:
•
Default RIP Configuration, page 41-21
•
Configuring Basic RIP Parameters, page 41-21
•
Configuring RIP Authentication, page 41-23
•
Configuring Summary Addresses and Split Horizon, page 41-23
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Default RIP Configuration
Table 41-4
Default RIP Configuration
Feature
Default Setting
Auto summary
Enabled.
Default-information originate
Disabled.
Default metric
Built-in; automatic metric translations.
IP RIP authentication key-chain No authentication.
Authentication mode: clear text.
IP RIP receive version
According to the version router configuration command.
IP RIP send version
According to the version router configuration command.
IP RIP triggered
According to the version router configuration command.
IP split horizon
Varies with media.
Neighbor
None defined.
Network
None specified.
Offset list
Disabled.
Output delay
0 milliseconds.
Timers basic
•
Update: 30 seconds.
•
Invalid: 180 seconds.
•
Hold-down: 180 seconds.
•
Flush: 240 seconds.
Validate-update-source
Enabled.
Version
Receives RIP Version 1 and 2 packets; sends Version 1 packets.
Configuring Basic RIP Parameters
Note
To configure RIP, you enable RIP routing for a network and optionally configure other parameters. On
the Catalyst 3750-X and 3560-X switches, RIP configuration commands are ignored until you configure
the network number.
Beginning in privileged EXEC mode, follow these steps to enable and configure RIP:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip routing
Enable IP routing. (Required only if IP routing is disabled.)
Step 3
router rip
Enable a RIP routing process, and enter router configuration mode.
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Configuring RIP
Step 4
Command
Purpose
network network number
Associate a network with a RIP routing process. You can specify multiple
network commands. RIP routing updates are sent and received through
interfaces only on these networks.
Note
You must configure a network number for the RIP commands to
take effect.
Step 5
neighbor ip-address
(Optional) Define a neighboring router with which to exchange routing
information. This step allows routing updates from RIP (normally a
broadcast protocol) to reach nonbroadcast networks.
Step 6
offset list [access-list number | name]
{in | out} offset [type number]
(Optional) Apply an offset list to routing metrics to increase incoming
and outgoing metrics to routes learned through RIP. You can limit the
offset list with an access list or an interface.
Step 7
timers basic update invalid holddown
flush
(Optional) Adjust routing protocol timers. Valid ranges for all timers are
0 to 4294967295 seconds.
•
update—The time between sending routing updates. The default is
30 seconds.
•
invalid—The timer after which a route is declared invalid. The
default is 180 seconds.
•
holddown—The time before a route is removed from the routing
table. The default is 180 seconds.
•
flush—The amount of time for which routing updates are postponed.
The default is 240 seconds.
Step 8
version {1 | 2}
(Optional) Configure the switch to receive and send only RIP Version 1
or RIP Version 2 packets. By default, the switch receives Version 1 and 2
but sends only Version 1.
You can also use the interface commands ip rip {send | receive} version
1 | 2 | 1 2} to control what versions are used for sending and receiving on
interfaces.
Step 9
no auto summary
(Optional) Disable automatic summarization. By default, the switch
summarizes subprefixes when crossing classful network boundaries.
Disable summarization (RIP Version 2 only) to advertise subnet and host
routing information to classful network boundaries.
Step 10
no validate-update-source
(Optional) Disable validation of the source IP address of incoming RIP
routing updates. By default, the switch validates the source IP address of
incoming RIP routing updates and discards the update if the source
address is not valid. Under normal circumstances, disabling this feature
is not recommended. However, if you have a router that is off-network
and you want to receive its updates, you can use this command.
Step 11
output-delay delay
(Optional) Add interpacket delay for RIP updates sent.
By default, packets in a multiple-packet RIP update have no delay added
between packets. If you are sending packets to a lower-speed device, you
can add an interpacket delay in the range of 8 to 50 milliseconds.
Step 12
end
Return to privileged EXEC mode.
Step 13
show ip protocols
Verify your entries.
Step 14
copy running-config startup-config
(Optional) Save your entries in the configuration file.
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Configuring RIP
To turn off the RIP routing process, use the no router rip global configuration command.
To display the parameters and current state of the active routing protocol process, use the show ip
protocols privileged EXEC command. Use the show ip rip database privileged EXEC command to
display summary address entries in the RIP database.
Configuring RIP Authentication
RIP Version 1 does not support authentication. If you are sending and receiving RIP Version 2 packets,
you can enable RIP authentication on an interface. The key chain specifies \the set of keys that can be
used on the interface. If a key chain is not configured, no authentication is performed, not even the
default. Therefore, you must also perform the tasks in the “Managing Authentication Keys” section on
page 41-104.
The switch supports two modes of authentication on interfaces for which RIP authentication is enabled:
plain text and MD5. The default is plain text.
Beginning in privileged EXEC mode, follow these steps to configure RIP authentication on an interface:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the
interface to configure.
Step 3
ip rip authentication key-chain name-of-chain
Enable RIP authentication.
Step 4
ip rip authentication mode [text | md5}
Configure the interface to use plain text authentication (the
default) or MD5 digest authentication.
Step 5
end
Return to privileged EXEC mode.
Step 6
show running-config interface [interface-id]
Verify your entries.
Step 7
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To restore clear text authentication, use the no ip rip authentication mode interface configuration
command. To prevent authentication, use the no ip rip authentication key-chain interface
configuration command.
Configuring Summary Addresses and Split Horizon
Routers connected to broadcast-type IP networks and using distance-vector routing protocols normally
use the split-horizon mechanism to reduce the possibility of routing loops. Split horizon blocks
information about routes from being advertised by a router on any interface from which that information
originated. This feature usually optimizes communication among multiple routers, especially when links
are broken.
Note
In general, disabling split horizon is not recommended unless you are certain that your application
requires it to properly advertise routes.
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Configuring RIP
If you want to configure an interface running RIP to advertise a summarized local IP address pool on a
network access server for dial-up clients, use the ip summary-address rip interface configuration
command.
Note
If split horizon is enabled, neither autosummary nor interface IP summary addresses are advertised.
Beginning in privileged EXEC mode, follow these steps to set an interface to advertise a summarized
local IP address and to disable split horizon on the interface:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the Layer 3
interface to configure.
Step 3
ip address ip-address subnet-mask
Configure the IP address and IP subnet.
Step 4
ip summary-address rip ip address ip-network mask Configure the IP address to be summarized and the IP
network mask.
Step 5
no ip split horizon
Disable split horizon on the interface.
Step 6
end
Return to privileged EXEC mode.
Step 7
show ip interface interface-id
Verify your entries.
Step 8
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To disable IP summarization, use the no ip summary-address rip router configuration command.
In this example, the major net is 10.0.0.0. The summary address 10.2.0.0 overrides the autosummary
address of 10.0.0.0 so that 10.2.0.0 is advertised out interface Gigabit Ethernet port 2, and 10.0.0.0 is
not advertised. In the example, if the interface is still in Layer 2 mode (the default), you must enter a no
switchport interface configuration command before entering the ip address interface configuration
command.
Note
If split horizon is enabled, neither autosummary nor interface summary addresses (those configured with
the ip summary-address rip router configuration command) are advertised.
Switch(config)# router rip
Switch(config-router)# interface gigabitethernet1/0/2
Switch(config-if)# ip address 10.1.5.1 255.255.255.0
Switch(config-if)# ip summary-address rip 10.2.0.0 255.255.0.0
Switch(config-if)# no ip split-horizon
Switch(config-if)# exit
Switch(config)# router rip
Switch(config-router)# network 10.0.0.0
Switch(config-router)# neighbor 2.2.2.2 peer-group mygroup
Switch(config-router)# end
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Configuring Split Horizon
Routers connected to broadcast-type IP networks and using distance-vector routing protocols normally
use the split-horizon mechanism to reduce the possibility of routing loops. Split horizon blocks
information about routes from being advertised by a router on any interface from which that information
originated. This feature can optimize communication among multiple routers, especially when links are
broken.
Note
In general, we do not recommend disabling split horizon unless you are certain that your application
requires it to properly advertise routes.
Beginning in privileged EXEC mode, follow these steps to disable split horizon on the interface:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the interface to
configure.
Step 3
ip address ip-address subnet-mask
Configure the IP address and IP subnet.
Step 4
no ip split-horizon
Disable split horizon on the interface.
Step 5
end
Return to privileged EXEC mode.
Step 6
show ip interface interface-id
Verify your entries.
Step 7
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To enable the split horizon mechanism, use the ip split-horizon interface configuration command.
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Configuring OSPF
Configuring OSPF
This section briefly describes how to configure Open Shortest Path First (OSPF). For a complete
description of the OSPF commands, see the “OSPF Commands” chapter of the Cisco IOS IP Command
Reference, Volume 2 of 3: Routing Protocols, Release 12.2.
Note
OSPF classifies different media into broadcast, nonbroadcast, and point-to-point networks. The switch
supports broadcast (Ethernet, Token Ring, and FDDI) and point-to-point networks (Ethernet interfaces
configured as point-to-point links).
OSPF is an Interior Gateway Protocol (IGP) designed expressly for IP networks, supporting IP
subnetting and tagging of externally derived routing information. OSPF also allows packet
authentication and uses IP multicast when sending and receiving packets. The Cisco implementation
supports RFC 1253, OSPF management information base (MIB).
The Cisco implementation conforms to the OSPF Version 2 specifications with these key features:
•
Definition of stub areas is supported.
•
Routes learned through any IP routing protocol can be redistributed into another IP routing protocol.
At the intradomain level, this means that OSPF can import routes learned through EIGRP and RIP.
OSPF routes can also be exported into RIP.
•
Plain text and MD5 authentication among neighboring routers within an area is supported.
•
Configurable routing interface parameters include interface output cost, retransmission interval,
interface transmit delay, router priority, router dead and hello intervals, and authentication key.
•
Virtual links are supported.
•
Not-so-stubby-areas (NSSAs) per RFC 1587are supported.
OSPF typically requires coordination among many internal routers, area border routers (ABRs)
connected to multiple areas, and autonomous system boundary routers (ASBRs). The minimum
configuration would use all default parameter values, no authentication, and interfaces assigned to areas.
If you customize your environment, you must ensure coordinated configuration of all routers.
These sections contain this configuration information:
•
Default OSPF Configuration, page 41-27
•
Configuring Basic OSPF Parameters, page 41-30
•
Configuring OSPF Interfaces, page 41-30
•
Configuring OSPF Area Parameters, page 41-32
•
Configuring Other OSPF Parameters, page 41-33
•
Changing LSA Group Pacing, page 41-35
•
Configuring a Loopback Interface, page 41-35
•
Monitoring OSPF, page 41-36
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Default OSPF Configuration
Table 41-5
Default OSPF Configuration
Feature
Default Setting
Interface parameters
Cost: No default cost predefined.
Retransmit interval: 5 seconds.
Transmit delay: 1 second.
Priority: 1.
Hello interval: 10 seconds.
Dead interval: 4 times the hello interval.
No authentication.
No password specified.
MD5 authentication disabled.
Area
Authentication type: 0 (no authentication).
Default cost: 1.
Range: Disabled.
Stub: No stub area defined.
NSSA: No NSSA area defined.
Auto cost
100 Mb/s.
Default-information originate
Disabled. When enabled, the default metric setting is 10, and the external route type
default is Type 2.
Default metric
Built-in, automatic metric translation, as appropriate for each routing protocol.
Distance OSPF
dist1 (all routes within an area): 110.
dist2 (all routes from one area to another): 110.
and dist3 (routes from other routing domains): 110.
OSPF database filter
Disabled. All outgoing link-state advertisements (LSAs) are flooded to the interface.
IP OSPF name lookup
Disabled.
Log adjacency changes
Enabled.
Neighbor
None specified.
Neighbor database filter
Disabled. All outgoing LSAs are flooded to the neighbor.
Network area
Disabled.
1
NSF awareness
Enabled2. Allows Layer 3 switches to continue forwarding packets from a neighboring
NSF-capable router during hardware or software changes.
NSF capability
Disabled.
Note
The switch stack supports OSPF NSF-capable routing for IPv4.
Router ID
No OSPF routing process defined.
Summary address
Disabled.
Timers LSA group pacing
240 seconds.
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Table 41-5
Default OSPF Configuration (continued)
Feature
Default Setting
Timers shortest path first (spf)
spf delay: 5 seconds.; spf-holdtime: 10 seconds.
Virtual link
No area ID or router ID defined.
Hello interval: 10 seconds.
Retransmit interval: 5 seconds.
Transmit delay: 1 second.
Dead interval: 40 seconds.
Authentication key: no key predefined.
Message-digest key (MD5): no key predefined.
1. NSF = Nonstop forwarding.
2. OSPF NSF awareness is enabled for IPv4 on Catalyst 3750-E and 3560-E switches running the IP services feature set.
OSPF for Routed Access
With Cisco IOS Release 12.2(55)SE, the IP Base image supports OSPF for routed access. The IP services
image is required if you need multiple OSPFv2 and OSPFv3 instances without route restrictions.
Additionally, the IP services image is required to enable the multi-VRF-CE feature.
OSPF for Routed Access is specifically designed so that you can extend Layer 3 routing capabilities to
the wiring closet.
Note
OSPF for Routed Access supports only one OSPFv2 and one OSPFv3 instance with a combined total of
200 dynamically learned routes. The IP Base image provides OSPF for routed access.
However, these restrictions are not enforced in this release.
With the typical topology (hub and spoke) in a campus environment, where the wiring closets (spokes)
are connected to the distribution switch (hub) that forwards all nonlocal traffic to the distribution layer,
the wiring closet switch need not hold a complete routing table. A best practice design, where the
distribution switch sends a default route to the wiring closet switch to reach interarea and external routes
(OSPF stub or totally stub area configuration) should be used when OSPF for Routed Access is used in
the wiring closet.
For more details, see the “High Availability Campus Network Design—Routed Access Layer using
EIGRP or OSPF” document.
OSPF Nonstop Forwarding
The switch or switch stack supports two levels of nonstop forwarding (NSF):
•
OSPF NSF Awareness, page 41-29
•
OSPF NSF Capability, page 41-29
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OSPF NSF Awareness
The IP-services feature set supports OSPF NSF Awareness supported for IPv4. When the neighboring
router is NSF-capable, the Layer 3 switch continues to forward packets from the neighboring router
during the interval between the primary Route Processor (RP) in a router crashing and the backup RP
taking over, or while the primary RP is manually reloaded for a non-disruptive software upgrade.
This feature cannot be disabled. For more information on this feature, see OSPF Nonstop Forwarding
(NSF) Awareness at this URL:
http://www.cisco.com/en/US/docs/ios/12_2t/12_2t15/feature/guide/ftosnsfa.html
OSPF NSF Capability
The IP-services feature set also supports OSPF NSF-capable routing for IPv4 for better convergence and
lower traffic loss following a stack master change. When a stack master change occurs in an OSPF
NSF-capable stack, the new stack master must do two things to resynchronize its link-state database with
its OSFP neighbors:
•
Release the available OSPF neighbors on the network without resetting the neighbor relationship.
•
Reacquire the contents of the link-state database for the network.
After a stack master change, the new master sends an OSPF NSF signal to neighboring NSF-aware
devices. A device recognizes this signal to mean that it should not reset the neighbor relationship with
the stack. As the NSF-capable stack master receives signals from other routes on the network, it begins
to rebuild its neighbor list.
When the neighbor relationships are reestablished, the NSF-capable stack master resynchronizes its
database with its NSF-aware neighbors, and routing information is exchanged between the OSPF
neighbors. The new stack master uses this routing information to remove stale routes, to update the
routing information database (RIB), and to update the forwarding information base (FIB) with the new
information. The OSPF protocols then fully converge.
Note
OSPF NSF requires that all neighbor networking devices be NSF-aware. If an NSF-capable router
discovers non-NSF aware neighbors on a network segment, it disables NSF capabilities for that segment.
Other network segments where all devices are NSF-aware or NSF-capable continue to provide NSF
capabilities.
Use the nsf OSPF routing configuration command to enable OSPF NSF routing. Use the show ip ospf
privileged EXEC command to verify that it is enabled.
For more information, see Cisco Nonstop Forwarding at this URL:
http://www.cisco.com/en/US/docs/ios/ha/configuration/guide/ha-nonstp_fwdg.html
Note
NSF is not supported on interfaces configured for Hot Standby Router Protocol (HSRP).
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Configuring OSPF
Configuring Basic OSPF Parameters
Enabling OSPF requires that you create an OSPF routing process, specify the range of IP addresses to
be associated with the routing process, and assign area IDs to be associated with that range.
Beginning in privileged EXEC mode, follow these steps to enable OSPF:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router ospf process-id
Enable OSPF routing, and enter router configuration mode. The
process ID is an internally used identification parameter that is
locally assigned and can be any positive integer. Each OSPF
routing process has a unique value.
Note
OSPF for Routed Access supports only one OSPFv2 and
one OSPFv3 instance with a maximum number of 200
dynamically learned routes.
Step 3
nsf
(Optional) Enable NSF operations for OSPF.
Step 4
network address wildcard-mask area area-id
Define an interface on which OSPF runs and the area ID for that
interface. You can use the wildcard-mask to use a single
command to define one or more multiple interfaces to be
associated with a specific OSPF area. The area ID can be a
decimal value or an IP address.
Step 5
end
Return to privileged EXEC mode.
Step 6
show ip protocols
Verify your entries.
Step 7
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To end an OSPF routing process, use the no router ospf process-id global configuration command.
This example shows how to configure an OSPF routing process and assign it a process number of 109:
Switch(config)# router ospf 109
Switch(config-router)# network 131.108.0.0 255.255.255.0 area 24
Configuring OSPF Interfaces
You can use the ip ospf interface configuration commands to modify interface-specific OSPF
parameters. You are not required to modify any of these parameters, but some interface parameters (hello
interval, dead interval, and authentication key) must be consistent across all routers in an attached
network. If you modify these parameters, be sure all routers in the network have compatible values.
Note
The ip ospf interface configuration commands are all optional.
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Beginning in privileged EXEC mode, follow these steps to modify OSPF interface parameters:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the Layer 3 interface
to configure.
Step 3
ip ospf cost
(Optional) Explicitly specify the cost of sending a packet on the
interface.
Step 4
ip ospf retransmit-interval seconds
(Optional) Specify the number of seconds between link state
advertisement transmissions. The range is 1 to 65535 seconds. The
default is 5 seconds.
Step 5
ip ospf transmit-delay seconds
(Optional) Set the estimated number of seconds to wait before
sending a link state update packet. The range is 1 to 65535 seconds.
The default is 1 second.
Step 6
ip ospf priority number
(Optional) Set priority to help find the OSPF designated router for a
network. The range is from 0 to 255. The default is 1.
Step 7
ip ospf hello-interval seconds
(Optional) Set the number of seconds between hello packets sent on
an OSPF interface. The value must be the same for all nodes on a
network. The range is 1 to 65535 seconds. The default is 10 seconds.
Step 8
ip ospf dead-interval seconds
(Optional) Set the number of seconds after the last device hello
packet was seen before its neighbors declare the OSPF router to be
down. The value must be the same for all nodes on a network. The
range is 1 to 65535 seconds. The default is 4 times the hello interval.
Step 9
ip ospf authentication-key key
(Optional) Assign a password to be used by neighboring OSPF
routers. The password can be any string of keyboard-entered
characters up to 8 bytes in length. All neighboring routers on the
same network must have the same password to exchange OSPF
information.
Step 10
ip ospf message digest-key keyid md5 key
(Optional) Enable MDS authentication.
•
keyid—An identifier from 1 to 255.
•
key—An alphanumeric password of up to 16 bytes.
Step 11
ip ospf database-filter all out
(Optional) Block flooding of OSPF LSA packets to the interface. By
default, OSPF floods new LSAs over all interfaces in the same area,
except the interface on which the LSA arrives.
Step 12
end
Return to privileged EXEC mode.
Step 13
show ip ospf interface [interface-name]
Display OSPF-related interface information.
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Step 14
Command
Purpose
show ip ospf neighbor detail
Display NSF awareness status of neighbor switch. The output
matches one of these examples:
•
Options is 0x52
LLS Options is 0x1 (LR)
When both of these lines appear, the neighbor switch is NSF
aware.
•
Step 15
copy running-config startup-config
Options is 0x42—This means the neighbor switch is not NSF
aware.
(Optional) Save your entries in the configuration file.
Use the no form of these commands to remove the configured parameter value or return to the
default value.
Configuring OSPF Area Parameters
You can optionally configure several OSPF area parameters. These parameters include authentication for
password-based protection against unauthorized access to an area, stub areas, and not-so-stubby-areas
(NSSAs). Stub areas are areas into which information on external routes is not sent. Instead, the area
border router (ABR) generates a default external route into the stub area for destinations outside the
autonomous system (AS). An NSSA does not flood all LSAs from the core into the area, but can import
AS external routes within the area by redistribution.
Route summarization is the consolidation of advertised addresses into a single summary route to be
advertised by other areas. If network numbers are contiguous, you can use the area range router
configuration command to configure the ABR to advertise a summary route that covers all networks in
the range.
Note
The OSPF area router configuration commands are all optional.
Beginning in privileged EXEC mode, follow these steps to configure area parameters:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router ospf process-id
Enable OSPF routing, and enter router configuration mode.
Step 3
area area-id authentication
(Optional) Allow password-based protection against unauthorized
access to the identified area. The identifier can be either a decimal
value or an IP address.
Step 4
area area-id authentication message-digest (Optional) Enable MD5 authentication on the area.
Step 5
area area-id stub [no-summary]
(Optional) Define an area as a stub area. The no-summary keyword
prevents an ABR from sending summary link advertisements into the
stub area.
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Step 6
Command
Purpose
area area-id nssa [no-redistribution]
[default-information-originate]
[no-summary]
(Optional) Defines an area as a not-so-stubby-area. Every router
within the same area must agree that the area is NSSA. Select one of
these keywords:
•
no-redistribution—Select when the router is an NSSA ABR and
you want the redistribute command to import routes into normal
areas, but not into the NSSA.
•
default-information-originate—Select on an ABR to allow
importing type 7 LSAs into the NSSA.
•
no-redistribution—Select to not send summary LSAs into the
NSSA.
Step 7
area area-id range address mask
(Optional) Specify an address range for which a single route is
advertised. Use this command only with area border routers.
Step 8
end
Return to privileged EXEC mode.
Step 9
show ip ospf [process-id]
Display information about the OSPF routing process in general or for
a specific process ID to verify configuration.
show ip ospf [process-id [area-id]] database Display lists of information related to the OSPF database for a
specific router.
Step 10
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no form of these commands to remove the configured parameter value or to return to the
default value.
Configuring Other OSPF Parameters
You can optionally configure other OSPF parameters in router configuration mode.
•
Route summarization: When redistributing routes from other protocols as described in the “Using
Route Maps to Redistribute Routing Information” section on page 41-94, each route is advertised
individually in an external LSA. To help decrease the size of the OSPF link state database, you can
use the summary-address router configuration command to advertise a single router for all the
redistributed routes included in a specified network address and mask.
•
Virtual links: In OSPF, all areas must be connected to a backbone area. You can establish a virtual
link in case of a backbone-continuity break by configuring two Area Border Routers as endpoints
of a virtual link. Configuration information includes the identity of the other virtual endpoint (the
other ABR) and the nonbackbone link that the two routers have in common (the transit area). Virtual
links cannot be configured through a stub area.
•
Default route: When you specifically configure redistribution of routes into an OSPF routing
domain, the route automatically becomes an autonomous system boundary router (ASBR). You can
force the ASBR to generate a default route into the OSPF routing domain.
•
Domain Name Server (DNS) names for use in all OSPF show privileged EXEC command displays
makes it easier to identify a router than displaying it by router ID or neighbor ID.
•
Default Metrics: OSPF calculates the OSPF metric for an interface according to the bandwidth of
the interface. The metric is calculated as ref-bw divided by bandwidth, where ref is 10 by default,
and bandwidth (bw) is specified by the bandwidth interface configuration command. For multiple
links with high bandwidth, you can specify a larger number to differentiate the cost on those links.
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•
Administrative distance is a rating of the trustworthiness of a routing information source, an integer
between 0 and 255, with a higher value meaning a lower trust rating. An administrative distance of
255 means the routing information source cannot be trusted at all and should be ignored. OSPF uses
three different administrative distances: routes within an area (interarea), routes to another area
(interarea), and routes from another routing domain learned through redistribution (external). You
can change any of the distance values.
•
Passive interfaces: Because interfaces between two devices on an Ethernet represent only one
network segment, to prevent OSPF from sending hello packets for the sending interface, you must
configure the sending device to be a passive interface. Both devices can identify each other through
the hello packet for the receiving interface.
•
Route calculation timers: You can configure the delay time between when OSPF receives a topology
change and when it starts the shortest path first (SPF) calculation and the hold time between two
SPF calculations.
•
Log neighbor changes: You can configure the router to send a syslog message when an OSPF
neighbor state changes, providing a high-level view of changes in the router.
Beginning in privileged EXEC mode, follow these steps to configure these OSPF parameters:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router ospf process-id
Enable OSPF routing, and enter router configuration mode.
Step 3
summary-address address mask
(Optional) Specify an address and IP subnet mask for redistributed
routes so that only one summary route is advertised.
Step 4
area area-id virtual-link router-id
[hello-interval seconds]
[retransmit-interval seconds] [trans]
[[authentication-key key] |
message-digest-key keyid md5 key]]
(Optional) Establish a virtual link and set its parameters. See the
“Configuring OSPF Interfaces” section on page 41-30 for parameter
definitions and Table 41-5 on page 41-27 for virtual link defaults.
Step 5
default-information originate [always]
[metric metric-value] [metric-type
type-value] [route-map map-name]
(Optional) Force the ASBR to generate a default route into the OSPF
routing domain. Parameters are all optional.
Step 6
ip ospf name-lookup
(Optional) Configure DNS name lookup. The default is disabled.
Step 7
ip auto-cost reference-bandwidth ref-bw
(Optional) Specify an address range for which a single route will be
advertised. Use this command only with area border routers.
Step 8
distance ospf {[inter-area dist1] [inter-area (Optional) Change the OSPF distance values. The default distance
dist2] [external dist3]}
for each type of route is 110. The range is 1 to 255.
Step 9
passive-interface type number
(Optional) Suppress the sending of hello packets through the
specified interface.
Step 10
timers throttle spf spf-delay spf-holdtime
spf-wait
(Optional) Configure route calculation timers.
Step 11
ospf log-adj-changes
•
spf-delay—Delay between receiving a change to SPF
calculation. The range is from 1 to 600000. miliseconds.
•
spf-holdtime—Delay between first and second SPF calculation.
The range is form 1 to 600000 in milliseconds.
•
spf-wait—Maximum wait time in milliseconds for SPF
calculations. The range is from 1 to 600000 in milliseconds.
(Optional) Send syslog message when a neighbor state changes.
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Command
Purpose
Step 12
end
Return to privileged EXEC mode.
Step 13
show ip ospf [process-id [area-id]] database Display lists of information related to the OSPF database for a
specific router. For some of the keyword options, see the “Monitoring
OSPF” section on page 41-36.
Step 14
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Changing LSA Group Pacing
The OSPF LSA group pacing feature allows the router to group OSPF LSAs and pace the refreshing,
check-summing, and aging functions for more efficient router use. This feature is enabled by default with
a 4-minute default pacing interval, and you will not usually need to modify this parameter. The optimum
group pacing interval is inversely proportional to the number of LSAs the router is refreshing,
check-summing, and aging. For example, if you have approximately 10,000 LSAs in the database,
decreasing the pacing interval would benefit you. If you have a very small database (40 to 100 LSAs),
increasing the pacing interval to 10 to 20 minutes might benefit you slightly.
Beginning in privileged EXEC mode, follow these steps to configure OSPF LSA pacing:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router ospf process-id
Enable OSPF routing, and enter router configuration mode.
Step 3
timers lsa-group-pacing seconds
Change the group pacing of LSAs.
Step 4
end
Return to privileged EXEC mode.
Step 5
show running-config
Verify your entries.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To return to the default value, use the no timers lsa-group-pacing router configuration command.
Configuring a Loopback Interface
OSPF uses the highest IP address configured on the interfaces as its router ID. If this interface is down
or removed, the OSPF process must recalculate a new router ID and resend all its routing information
out its interfaces. If a loopback interface is configured with an IP address, OSPF uses this IP address as
its router ID, even if other interfaces have higher IP addresses. Because loopback interfaces never fail,
this provides greater stability. OSPF automatically prefers a loopback interface over other interfaces, and
it chooses the highest IP address among all loopback interfaces.
Beginning in privileged EXEC mode, follow these steps to configure a loopback interface:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface loopback 0
Create a loopback interface, and enter interface configuration mode.
Step 3
ip address address mask
Assign an IP address to this interface.
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Command
Purpose
Step 4
end
Return to privileged EXEC mode.
Step 5
show ip interface
Verify your entries.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no interface loopback 0 global configuration command to disable the loopback interface.
Monitoring OSPF
You can display specific statistics such as the contents of IP routing tables, caches, and databases.
Table 41-6 lists some of the privileged EXEC commands for displaying statistics. For more show ip ospf
database privileged EXEC command options and for explanations of fields in the resulting display, see
the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.
Table 41-6
Show IP OSPF Statistics Commands
Command
Purpose
show ip ospf [process-id]
Display general information about OSPF routing
processes.
show ip ospf [process-id] database [router] [link-state-id]
Display lists of information related to the OSPF
database.
show ip ospf [process-id] database [router] [self-originate]
show ip ospf [process-id] database [router] [adv-router [ip-address]]
show ip ospf [process-id] database [network] [link-state-id]
show ip ospf [process-id] database [summary] [link-state-id]
show ip ospf [process-id] database [asbr-summary] [link-state-id]
show ip ospf [process-id] database [external] [link-state-id]
show ip ospf [process-id area-id] database [database-summary]
show ip ospf border-routes
Display the internal OSPF routing ABR and ASBR
table entries.
show ip ospf interface [interface-name]
Display OSPF-related interface information.
show ip ospf neighbor [interface-name] [neighbor-id] detail
Display OSPF interface neighbor information.
show ip ospf virtual-links
Display OSPF-related virtual links information.
Configuring EIGRP
Enhanced IGRP (EIGRP) is a Cisco proprietary enhanced version of the IGRP. EIGRP uses the same
distance vector algorithm and distance information as IGRP; however, the convergence properties and
the operating efficiency of EIGRP are significantly improved.
The convergence technology employs an algorithm referred to as the Diffusing Update Algorithm
(DUAL), which guarantees loop-free operation at every instant throughout a route computation and
allows all devices involved in a topology change to synchronize at the same time. Routers that are not
affected by topology changes are not involved in recomputations.
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IP EIGRP provides increased network width. With RIP, the largest possible width of your network is
15 hops. Because the EIGRP metric is large enough to support thousands of hops, the only barrier to
expanding the network is the transport-layer hop counter. EIGRP increments the transport control field
only when an IP packet has traversed 15 routers and the next hop to the destination was learned through
EIGRP. When a RIP route is used as the next hop to the destination, the transport control field is
incremented as usual.
EIGRP offers these features:
•
Fast convergence.
•
Incremental updates when the state of a destination changes, instead of sending the entire contents
of the routing table, minimizing the bandwidth required for EIGRP packets.
•
Less CPU usage because full update packets need not be processed each time they are received.
•
Protocol-independent neighbor discovery mechanism to learn about neighboring routers.
•
Variable-length subnet masks (VLSMs).
•
Arbitrary route summarization.
•
EIGRP scales to large networks.
EIGRP has these four basic components:
•
Neighbor discovery and recovery is the process that routers use to dynamically learn of other routers
on their directly attached networks. Routers must also discover when their neighbors become
unreachable or inoperative. Neighbor discovery and recovery is achieved with low overhead by
periodically sending small hello packets. As long as hello packets are received, the Cisco IOS
software can learn that a neighbor is alive and functioning. When this status is determined, the
neighboring routers can exchange routing information.
•
The reliable transport protocol is responsible for guaranteed, ordered delivery of EIGRP packets to
all neighbors. It supports intermixed transmission of multicast and unicast packets. Some EIGRP
packets must be sent reliably, and others need not be. For efficiency, reliability is provided only
when necessary. For example, on a multiaccess network that has multicast capabilities (such as
Ethernet), it is not necessary to send hellos reliably to all neighbors individually. Therefore, EIGRP
sends a single multicast hello with an indication in the packet informing the receivers that the packet
need not be acknowledged. Other types of packets (such as updates) require acknowledgment, which
is shown in the packet. The reliable transport has a provision to send multicast packets quickly when
there are unacknowledged packets pending. Doing so helps ensure that convergence time remains
low in the presence of varying speed links.
•
The DUAL finite state machine embodies the decision process for all route computations. It tracks
all routes advertised by all neighbors. DUAL uses the distance information (known as a metric) to
select efficient, loop-free paths. DUAL selects routes to be inserted into a routing table based on
feasible successors. A successor is a neighboring router used for packet forwarding that has a
least-cost path to a destination that is guaranteed not to be part of a routing loop. When there are no
feasible successors, but there are neighbors advertising the destination, a recomputation must occur.
This is the process whereby a new successor is determined. The amount of time it takes to recompute
the route affects the convergence time. Recomputation is processor-intensive; it is advantageous to
avoid recomputation if it is not necessary. When a topology change occurs, DUAL tests for feasible
successors. If there are feasible successors, it uses any it finds to avoid unnecessary recomputation.
•
The protocol-dependent modules are responsible for network layer protocol-specific tasks. An
example is the IP EIGRP module, which is responsible for sending and receiving EIGRP packets
that are encapsulated in IP. It is also responsible for parsing EIGRP packets and informing DUAL
of the new information received. EIGRP asks DUAL to make routing decisions, but the results are
stored in the IP routing table. EIGRP is also responsible for redistributing routes learned by other
IP routing protocols.
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Configuring EIGRP
These sections contain this configuration information:
Note
•
Default EIGRP Configuration, page 41-38
•
Configuring Basic EIGRP Parameters, page 41-40
•
Configuring EIGRP Interfaces, page 41-41
•
Configuring EIGRP Route Authentication, page 41-42
•
EIGRP Stub Routing, page 41-43
•
Monitoring and Maintaining EIGRP, page 41-44
To enable EIGRP, the switch or stack master must be running the IP services feature set.
Default EIGRP Configuration
Table 41-7
Default EIGRP Configuration
Feature
Default Setting
Auto summary
Enabled. Subprefixes are summarized to the classful network boundary when crossing
classful network boundaries.
Default-information
Exterior routes are accepted and default information is passed between EIGRP processes
when doing redistribution.
Default metric
Only connected routes and interface static routes can be redistributed without a default
metric. The metric includes:
Distance
•
Bandwidth: 0 or greater kb/s.
•
Delay (tens of microseconds): 0 or any positive number that is a multiple of 39.1
nanoseconds.
•
Reliability: any number between 0 and 255 (255 means 100 percent reliability).
•
Loading: effective bandwidth as a number between 0 and 255 (255 is 100 percent
loading).
•
MTU: maximum transmission unit size of the route in bytes. 0 or any positive integer.
Internal distance: 90.
External distance: 170.
EIGRP log-neighbor changes
Disabled. No adjacency changes logged.
IP authentication key-chain
No authentication provided.
IP authentication mode
No authentication provided.
IP bandwidth-percent
50 percent.
IP hello interval
For low-speed nonbroadcast multiaccess (NBMA) networks: 60 seconds; all other
networks: 5 seconds.
IP hold-time
For low-speed NBMA networks: 180 seconds; all other networks: 15 seconds.
IP split-horizon
Enabled.
IP summary address
No summary aggregate addresses are predefined.
Metric weights
tos: 0; k1 and k3: 1; k2, k4, and k5: 0
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Table 41-7
Default EIGRP Configuration (continued)
Feature
Default Setting
Network
None specified.
1
NSF Awareness
Enabled2. Allows Layer 3 switches to continue forwarding packets from a neighboring
NSF-capable router during hardware or software changes.
NSF capability
Disabled.
Note
The switch supports EIGRP NSF-capable routing for IPv4.
Offset-list
Disabled.
Router EIGRP
Disabled.
Set metric
No metric set in the route map.
Traffic-share
Distributed proportionately to the ratios of the metrics.
Variance
1 (equal-cost load-balancing).
1. NSF = Nonstop Forwarding
2. EIGRP NSF awareness is enabled for IPv4 on switches running the IP services feature set.
To create an EIGRP routing process, you must enable EIGRP and associate networks. EIGRP sends
updates to the interfaces in the specified networks. If you do not specify an interface network, it is not
advertised in any EIGRP update.
Note
If you have routers on your network that are configured for IGRP, and you want to change to EIGRP, you
must designate transition routers that have both IGRP and EIGRP configured. In these cases, perform
Steps 1 through 3 in the next section and also see the “Configuring Split Horizon” section on page 41-25.
You must use the same AS number for routes to be automatically redistributed.
EIGRP Nonstop Forwarding
The switch stack supports two levels of EIGRP nonstop forwarding:
•
EIGRP NSF Awareness, page 41-39
•
EIGRP NSF Capability, page 41-39
EIGRP NSF Awareness
The IP-services feature set supports EIGRP NSF Awareness for IPv4. When the neighboring router is
NSF-capable, the Layer 3 switch continues to forward packets from the neighboring router during the
interval between the primary Route Processor (RP) in a router failing and the backup RP taking over, or
while the primary RP is manually reloaded for a nondisruptive software upgrade.
This feature cannot be disabled. For more information on this feature, see the “EIGRP Nonstop
Forwarding (NSF) Awareness” section of the Cisco IOS IP Routing Protocols Configuration Guide,
Release 12.4.
EIGRP NSF Capability
The IP-services feature set also supports EIGRP NSF-capable routing for IPv4 for better convergence
and lower traffic loss following a stack master change. When an EIGRP NSF-capable stack master
restarts or a new stack master starts up and NSF restarts, the switch has no neighbors, and the topology
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Configuring EIGRP
table is empty. The switch must bring up the interfaces, reacquire neighbors, and rebuild the topology
and routing tables without interrupting the traffic directed toward the switch stack. EIGRP peer routers
maintain the routes learned from the new stack master and continue forwarding traffic through the NSF
restart process.
To prevent an adjacency reset by the neighbors, the new stack master uses a new Restart (RS) bit in the
EIGRP packet header to show the restart. When the neighbor receives this, it synchronizes the stack in
its peer list and maintains the adjacency with the stack. The neighbor then sends its topology table to the
stack master with the RS bit set to show that it is NSF-aware and is aiding the new stack master.
If at least one of the stack peer neighbors is NSF-aware, the stack master receives updates and rebuilds
its database. Each NSF-aware neighbor sends an end of table (EOT) marker in the last update packet to
mark the end of the table content. The stack master recognizes the convergence when it receives the EOT
marker, and it then begins sending updates. When the stack master has received all EOT markers from
its neighbors or when the NSF converge timer expires, EIGRP notifies the routing information database
(RIB) of convergence and floods its topology table to all NSF-aware peers.
Note
NSF is not supported on interfaces configured for Hot Standby Router Protocol (HSRP).
Use the nsf EIGRP routing configuration command to enable EIGRP NSF routing. Use the show ip
protocols privileged EXEC command to verify that NSF is enabled on the device. See the command
reference for this release for information about the nsf command.
Configuring Basic EIGRP Parameters
Beginning in privileged EXEC mode, follow these steps to configure EIGRP. Configuring the routing
process is required; other steps are optional:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router eigrp autonomous-system
Enable an EIGRP routing process, and enter router configuration
mode. The AS number identifies the routes to other EIGRP routers
and is used to tag routing information.
Step 3
nsf
(Optional) Enable EIGRP NSF. Enter this command on the stack
master and on all of its peers.
Step 4
network network-number
Associate networks with an EIGRP routing process. EIGRP sends
updates to the interfaces in the specified networks.
Step 5
eigrp log-neighbor-changes
(Optional) Enable logging of EIGRP neighbor changes to monitor
routing system stability.
Step 6
metric weights tos k1 k2 k3 k4 k5
(Optional) Adjust the EIGRP metric. Although the defaults have
been carefully set to provide excellent operation in most networks,
you can adjust them.
Caution
Setting metrics is complex and is not recommended
without guidance from an experienced network
designer.
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Command
Purpose
Step 7
offset list [access-list number | name] {in | out} (Optional) Apply an offset list to routing metrics to increase
offset [type number]
incoming and outgoing metrics to routes learned through EIGRP.
You can limit the offset list with an access list or an interface.
Step 8
no auto-summary
(Optional) Disable automatic summarization of subnet routes into
network-level routes.
Step 9
ip summary-address eigrp
autonomous-system-number address mask
(Optional) Configure a summary aggregate.
Step 10
end
Return to privileged EXEC mode.
Step 11
show ip protocols
Verify your entries.
Step 12
show ip protocols
Verify your entries.
For NSF awareness, the output shows:
*** IP Routing is NSF aware ***
EIGRP NSF enabled
Step 13
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no forms of these commands to disable the feature or return the setting to the default value.
Configuring EIGRP Interfaces
Other optional EIGRP parameters can be configured on an interface basis.
Beginning in privileged EXEC mode, follow these steps to configure EIGRP interfaces:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the Layer 3
interface to configure.
Step 3
ip bandwidth-percent eigrp percent
(Optional) Configure the percentage of bandwidth that can
be used by EIGRP on an interface. The default is 50 percent.
Step 4
ip summary-address eigrp
autonomous-system-number address mask
(Optional) Configure a summary aggregate address for a
specified interface (not usually necessary if auto-summary is
enabled).
Step 5
ip hello-interval eigrp autonomous-system-number
seconds
(Optional) Change the hello time interval for an EIGRP
routing process. The range is 1 to 65535 seconds. The
default is 60 seconds for low-speed NBMA networks and 5
seconds for all other networks.
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Step 6
Command
Purpose
ip hold-time eigrp autonomous-system-number
seconds
(Optional) Change the hold time interval for an EIGRP
routing process. The range is 1 to 65535 seconds. The
default is 180 seconds for low-speed NBMA networks and
15 seconds for all other networks.
Caution
Do not adjust the hold time without consulting
Cisco technical support.
Step 7
no ip split-horizon eigrp autonomous-system-number (Optional) Disable split horizon to allow route information
to be advertised by a router out any interface from which that
information originated.
Step 8
end
Return to privileged EXEC mode.
Step 9
show ip eigrp interface
Display which interfaces EIGRP is active on and
information about EIGRP relating to those interfaces.
Step 10
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no forms of these commands to disable the feature or return the setting to the default value.
Configuring EIGRP Route Authentication
EIGRP route authentication provides MD5 authentication of routing updates from the EIGRP routing
protocol to prevent the introduction of unauthorized or false routing messages from unapproved sources.
Beginning in privileged EXEC mode, follow these steps to enable authentication:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the
Layer 3 interface to configure.
Step 3
ip authentication mode eigrp autonomous-system md5
Enable MD5 authentication in IP EIGRP packets.
Step 4
ip authentication key-chain eigrp autonomous-system
key-chain
Enable authentication of IP EIGRP packets.
Step 5
exit
Return to global configuration mode.
Step 6
key chain name-of-chain
Identify a key chain and enter key-chain configuration
mode. Match the name configured in Step 4.
Step 7
key number
In key-chain configuration mode, identify the key
number.
Step 8
key-string text
In key-chain key configuration mode, identify the key
string.
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Command
Step 9
Purpose
accept-lifetime start-time {infinite | end-time | duration (Optional) Specify the time period during which the key
seconds}
can be received.
The start-time and end-time syntax can be either
hh:mm:ss Month date year or hh:mm:ss date Month
year. The default is forever with the default start-time
and the earliest acceptable date as January 1, 1993. The
default end-time and duration is infinite.
Step 10
send-lifetime start-time {infinite | end-time | duration
seconds}
(Optional) Specify the time period during which the key
can be sent.
The start-time and end-time syntax can be either
hh:mm:ss Month date year or hh:mm:ss date Month
year. The default is forever with the default start-time
and the earliest acceptable date as January 1, 1993. The
default end-time and duration is infinite.
Step 11
end
Return to privileged EXEC mode.
Step 12
show key chain
Display authentication key information.
Step 13
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no forms of these commands to disable the feature or to return the setting to the default value.
EIGRP Stub Routing
The EIGRP stub routing feature, available in all feature sets, reduces resource utilization by moving
routed traffic closer to the end user.
Note
The IP base feature set contains EIGRP stub routing capability, which only advertises connected or
summary routes from the routing tables to other switches in the network. The switch uses EIGRP stub
routing at the access layer to eliminate the need for other types of routing advertisements. For enhanced
capability and complete EIGRP routing, the switch must be running the IP services feature set.
On a switch running the IP base feature set, if you try to configure multi-VRF-CE and EIGRP stub
routing at the same time, the configuration is not allowed.
In a network using EIGRP stub routing, the only allowable route for IP traffic to the user is through a
switch that is configured with EIGRP stub routing. The switch sends the routed traffic to interfaces that
are configured as user interfaces or are connected to other devices.
When using EIGRP stub routing, you need to configure the distribution and remote routers to use EIGRP
and to configure only the switch as a stub. Only specified routes are propagated from the switch. The
switch responds to all queries for summaries, connected routes, and routing updates.
Any neighbor that receives a packet informing it of the stub status does not query the stub router for any
routes, and a router that has a stub peer does not query that peer. The stub router depends on the distribution
router to send the proper updates to all peers.
In Figure 41-4, switch B is configured as an EIGRP stub router. Switches A and C are connected to the rest
of the WAN. Switch B advertises connected, static, redistribution, and summary routes to switch A and C.
Switch B does not advertise any routes learned from switch A (and the reverse).
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Configuring BGP
Figure 41-4
EIGRP Stub Router Configuration
Routed to WAN
Switch B
Switch C
145776
Switch A
Host A
Host B
Host C
For more information about EIGRP stub routing, see “Configuring EIGRP Stub Routing” section of the
Cisco IOS IP Configuration Guide, Volume 2 of 3: Routing Protocols, Release 12.2.
Monitoring and Maintaining EIGRP
You can delete neighbors from the neighbor table. You can also display various EIGRP routing statistics.
Table 41-8 lists the privileged EXEC commands for deleting neighbors and displaying statistics. For
explanations of fields in the resulting display, see the Cisco IOS IP Command Reference, Volume 2 of 3:
Routing Protocols, Release 12.2.
Table 41-8
IP EIGRP Clear and Show Commands
Command
Purpose
clear ip eigrp neighbors [if-address | interface]
Delete neighbors from the neighbor table.
show ip eigrp interface [interface] [as number]
Display information about interfaces configured for EIGRP.
show ip eigrp neighbors [type-number]
Display EIGRP discovered neighbors.
show ip eigrp topology [autonomous-system-number] |
[[ip-address] mask]]
Display the EIGRP topology table for a given process.
show ip eigrp traffic [autonomous-system-number]
Display the number of packets sent and received for all or a
specified EIGRP process.
Configuring BGP
The Border Gateway Protocol (BGP) is an exterior gateway protocol used to set up an interdomain
routing system that guarantees the loop-free exchange of routing information between autonomous
systems. Autonomous systems are made up of routers that operate under the same administration and
that run Interior Gateway Protocols (IGPs), such as RIP or OSPF, within their boundaries and that
interconnect by using an Exterior Gateway Protocol (EGP). BGP Version 4 is the standard EGP for
interdomain routing in the Internet. The protocol is defined in RFCs 1163, 1267, and 1771. You can find
detailed information about BGP in Internet Routing Architectures, published by Cisco Press, and in the
“Configuring BGP” chapter in the Cisco IP and IP Routing Configuration Guide.
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For details about BGP commands and keywords, see the “IP Routing Protocols” part of the Cisco IOS
IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2. For a list of BGP commands
that are visible but not supported by the switch, see Appendix C, “Unsupported Commands in
Cisco IOS Release 12.2(55)SE.”
Routers that belong to the same autonomous system (AS) and that exchange BGP updates run internal
BGP (IBGP), and routers that belong to different autonomous systems and that exchange BGP updates
run external BGP (EBGP). Most configuration commands are the same for configuring EBGP and IBGP.
The difference is that the routing updates are exchanged either between autonomous systems (EBGP) or
within an AS (IBGP). Figure 41-5 shows a network that is running both EBGP and IBGP.
AS 100
EBGP, IBGP, and Multiple Autonomous Systems
Router A
129.213.1.2
192.208.10.1
EBGP
EBGP
129.213.1.1
Router B
AS 300
Router D
192.208.10.2
IBGP
175.220.212.1
Router C
175.220.1.2
AS 200
74775
Figure 41-5
Before exchanging information with an external AS, BGP ensures that networks within the AS can be
reached by defining internal BGP peering among routers within the AS and by redistributing BGP
routing information to IGPs that run within the AS, such as IGRP and OSPF.
Routers that run a BGP routing process are often referred to as BGP speakers. BGP uses the
Transmission Control Protocol (TCP) as its transport protocol (specifically port 179). Two BGP speakers
that have a TCP connection to each other for exchanging routing information are known as peers or
neighbors. In Figure 41-5, Routers A and B are BGP peers, as are Routers B and C and Routers C and
D. The routing information is a series of AS numbers that describe the full path to the destination
network. BGP uses this information to construct a loop-free map of autonomous systems.
The network has these characteristics:
•
Routers A and B are running EBGP, and Routers B and C are running IBGP. Note that the EBGP
peers are directly connected and that the IBGP peers are not. As long as there is an IGP running that
allows the two neighbors to reach one another, IBGP peers do not have to be directly connected.
•
All BGP speakers within an AS must establish a peer relationship with each other. That is, the BGP
speakers within an AS must be fully meshed logically. BGP4 provides two techniques that reduce
the requirement for a logical full mesh: confederations and route reflectors.
•
AS 200 is a transit AS for AS 100 and AS 300—that is, AS 200 is used to transfer packets between
AS 100 and AS 300.
BGP peers initially exchange their full BGP routing tables and then send only incremental updates. BGP
peers also exchange keepalive messages (to ensure that the connection is up) and notification messages
(in response to errors or special conditions).
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Configuring BGP
In BGP, each route consists of a network number, a list of autonomous systems that information has
passed through (the autonomous system path), and a list of other path attributes. The primary function
of a BGP system is to exchange network reachability information, including information about the list
of AS paths, with other BGP systems. This information can be used to determine AS connectivity, to
prune routing loops, and to enforce AS-level policy decisions.
A router or switch running Cisco IOS does not select or use an IBGP route unless it has a route available
to the next-hop router and it has received synchronization from an IGP (unless IGP synchronization is
disabled). When multiple routes are available, BGP bases its path selection on attribute values. See the
“Configuring BGP Decision Attributes” section on page 41-53 for information about BGP attributes.
BGP Version 4 supports classless interdomain routing (CIDR) so you can reduce the size of your routing
tables by creating aggregate routes, resulting in supernets. CIDR eliminates the concept of network
classes within BGP and supports the advertising of IP prefixes.
These sections contain this configuration information:
•
Default BGP Configuration, page 41-46
•
Enabling BGP Routing, page 41-49
•
Managing Routing Policy Changes, page 41-51
•
Configuring BGP Decision Attributes, page 41-53
•
Configuring BGP Filtering with Route Maps, page 41-55
•
Configuring BGP Filtering by Neighbor, page 41-55
•
Configuring Prefix Lists for BGP Filtering, page 41-57
•
Configuring BGP Community Filtering, page 41-58
•
Configuring BGP Neighbors and Peer Groups, page 41-59
•
Configuring Aggregate Addresses, page 41-61
•
Configuring Routing Domain Confederations, page 41-62
•
Configuring BGP Route Reflectors, page 41-62
•
Configuring Route Dampening, page 41-63
•
Monitoring and Maintaining BGP, page 41-64
For detailed descriptions of BGP configuration, see the “Configuring BGP” chapter in the “IP Routing
Protocols” part of the Cisco IOS IP Configuration Guide, Release 12.2. For details about specific
commands, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2.
For a list of BGP commands that are visible but not supported by the switch, see Appendix C,
“Unsupported Commands in Cisco IOS Release 12.2(55)SE.”
Default BGP Configuration
Table 41-9 shows the basic default BGP configuration. For the defaults for all characteristics, see the
specific commands in the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols,
Release 12.2.
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Table 41-9
Default BGP Configuration
Feature
Default Setting
Aggregate address
Disabled: None defined.
AS path access list
None defined.
Auto summary
Enabled.
Best path
BGP community list
BGP confederation identifier/peers
•
The router considers as-path in choosing a route and does not compare similar
routes from external BGP peers.
•
Compare router ID: Disabled.
•
Number: None defined. When you permit a value for the community number, the
list defaults to an implicit deny for everything else that has not been permitted.
•
Format: Cisco default format (32-bit number).
•
Identifier: None configured.
•
Peers: None identified.
BGP Fast external fallover
Enabled.
BGP local preference
100. The range is 0 to 4294967295 with the higher value preferred.
BGP network
None specified; no backdoor route advertised.
BGP route dampening
Disabled by default. When enabled:
BGP router ID
•
Half-life is 15 minutes.
•
Re-use is 750 (10-second increments).
•
Suppress is 2000 (10-second increments).
•
Max-suppress-time is 4 times half-life; 60 minutes.
The IP address of a loopback interface if one is configured or the highest IP address
configured for a physical interface on the router.
Default information originate
Disabled.
(protocol or network redistribution)
Default metric
Distance
Distribute list
Built-in, automatic metric translations.
•
External route administrative distance: 20 (acceptable values are from 1 to 255).
•
Internal route administrative distance: 200 (acceptable values are from 1 to 255).
•
Local route administrative distance: 200 (acceptable values are from 1 to 255).
•
In (filter networks received in updates): Disabled.
•
Out (suppress networks from being advertised in updates): Disabled.
Internal route redistribution
Disabled.
IP prefix list
None defined.
Multi exit discriminator (MED)
•
Always compare: Disabled. Does not compare MEDs for paths from neighbors in
different autonomous systems.
•
Best path compare: Disabled.
•
MED missing as worst path: Disabled.
•
Deterministic MED comparison is disabled.
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Table 41-9
Default BGP Configuration (continued)
Feature
Default Setting
Neighbor
•
Advertisement interval: 30 seconds for external peers; 5 seconds for internal peers.
•
Change logging: Enabled.
•
Conditional advertisement: Disabled.
•
Default originate: No default route is sent to the neighbor.
•
Description: None.
•
Distribute list: None defined.
•
External BGP multihop: Only directly connected neighbors are allowed.
•
Filter list: None used.
•
Maximum number of prefixes received: No limit.
•
Next hop (router as next hop for BGP neighbor): Disabled.
•
Password: Disabled.
•
Peer group: None defined; no members assigned.
•
Prefix list: None specified.
•
Remote AS (add entry to neighbor BGP table): No peers defined.
•
Private AS number removal: Disabled.
•
Route maps: None applied to a peer.
•
Send community attributes: None sent to neighbors.
•
Shutdown or soft reconfiguration: Not enabled.
•
Timers: keepalive: 60 seconds; holdtime: 180 seconds.
•
Update source: Best local address.
•
Version: BGP Version 4.
•
Weight: Routes learned through BGP peer: 0; routes sourced by the local router:
32768.
NSF1 Awareness
Disabled2. Allows Layer 3 switches to continue forwarding packets from a neighboring
NSF-capable router during hardware or software changes.
Route reflector
None configured.
Synchronization (BGP and IGP)
Enabled.
Table map update
Disabled.
Timers
Keepalive: 60 seconds; holdtime: 180 seconds.
1. NSF = Nonstop Forwarding
2. NSF Awareness can be enabled for IPv4 on switches with the IP services feature set by enabling Graceful Restart.
Nonstop Forwarding Awareness
The BGP NSF Awareness feature is supported for IPv4 in the IP services feature set. To enable this
feature with BGP routing, you need to enable Graceful Restart. When the neighboring router is
NSF-capable, and this feature is enabled, the Layer 3 switch continues to forward packets from the
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neighboring router during the interval between the primary Route Processor (RP) in a router failing and
the backup RP taking over, or while the primary RP is manually reloaded for a nondisruptive software
upgrade.
For more information, see the “BGP Nonstop Forwarding (NSF) Awareness” section of the Cisco IOS
IP Routing Protocols Configuration Guide, Release 12.4.
Enabling BGP Routing
To enable BGP routing, you establish a BGP routing process and define the local network. Because BGP
must completely recognize the relationships with its neighbors, you must also specify a BGP neighbor.
BGP supports two kinds of neighbors: internal and external. Internal neighbors are in the same AS;
external neighbors are in different autonomous systems. External neighbors are usually adjacent to each
other and share a subnet, but internal neighbors can be anywhere in the same AS.
The switch supports the use of private AS numbers, usually assigned by service providers and given to
systems whose routes are not advertised to external neighbors. The private AS numbers are from 64512
to 65535. You can configure external neighbors to remove private AS numbers from the AS path by using
the neighbor remove-private-as router configuration command. Then when an update is passed to an
external neighbor, if the AS path includes private AS numbers, these numbers are dropped.
If your AS will be passing traffic through it from another AS to a third AS, it is important to be consistent
about the routes it advertises. If BGP advertised a route before all routers in the network had learned
about the route through the IGP, the AS might receive traffic that some routers could not yet route. To
prevent this from happening, BGP must wait until the IGP has propagated information across the AS so
that BGP is synchronized with the IGP. Synchronization is enabled by default. If your AS does not pass
traffic from one AS to another AS, or if all routers in your autonomous systems are running BGP, you
can disable synchronization, which allows your network to carry fewer routes in the IGP and allows BGP
to converge more quickly.
Note
To enable BGP, the switch or stack master must be running the IP services feature set.
Beginning in privileged EXEC mode, follow these steps to enable BGP routing, establish a BGP routing
process, and specify a neighbor:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip routing
Enable IP routing (required only if IP routing is disabled).
Step 3
router bgp autonomous-system
Enable a BGP routing process, assign it an AS number, and
enter router configuration mode. The AS number can be from
1 to 65535, with 64512 to 65535 designated as private
autonomous numbers.
Step 4
network network-number [mask network-mask]
[route-map route-map-name]
Configure a network as local to this AS, and enter it in the BGP
table.
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Step 5
Command
Purpose
neighbor {ip-address | peer-group-name}
remote-as number
Add an entry to the BGP neighbor table specifying that the
neighbor identified by the IP address belongs to the specified
AS.
For EBGP, neighbors are usually directly connected, and the IP
address is the address of the interface at the other end of the
connection.
For IBGP, the IP address can be the address of any of the router
interfaces.
Step 6
neighbor {ip-address | peer-group-name}
remove-private-as
(Optional) Remove private AS numbers from the AS-path in
outbound routing updates.
Step 7
no synchronization
(Optional) Disable synchronization between BGP and an IGP.
Step 8
no auto-summary
(Optional) Disable automatic network summarization. By
default, when a subnet is redistributed from an IGP into BGP,
only the network route is inserted into the BGP table.
Step 9
bgp fast-external-fallover
(Optional) Automatically reset a BGP session when a link
between external neighbors goes down. By default, the session
is not immediately reset.
Step 10
bgp graceful-restart
(Optional) Enable NSF awareness on switch. By default, NSF
awareness is disabled.
Step 11
end
Return to privileged EXEC mode.
Step 12
show ip bgp network network-number
Verify the configuration.
or
show ip bgp neighbor
Verify that NSF awareness (Graceful Restart) is enabled on the
neighbor.
If NSF awareness is enabled on the switch and the neighbor,
this message appears:
Graceful Restart Capability: advertised and received
If NSF awareness is enabled on the switch, but not on the
neighbor, this message appears:
Graceful Restart Capability: advertised
Step 13
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no router bgp autonomous-system global configuration command to remove a BGP AS. Use the
no network network-number router configuration command to remove the network from the BGP table.
Use the no neighbor {ip-address | peer-group-name} remote-as number router configuration command
to remove a neighbor. Use the no neighbor {ip-address | peer-group-name} remove-private-as router
configuration command to include private AS numbers in updates to a neighbor. Use the
synchronization router configuration command to re-enable synchronization.
These examples show how to configure BGP on the routers in Figure 41-5.
Router A:
Switch(config)# router bgp 100
Switch(config-router)# neighbor 129.213.1.1 remote-as 200
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Router B:
Switch(config)# router bgp 200
Switch(config-router)# neighbor 129.213.1.2 remote-as 100
Switch(config-router)# neighbor 175.220.1.2 remote-as 200
Router C:
Switch(config)# router bgp 200
Switch(config-router)# neighbor 175.220.212.1 remote-as 200
Switch(config-router)# neighbor 192.208.10.1 remote-as 300
Router D:
Switch(config)# router bgp 300
Switch(config-router)# neighbor 192.208.10.2 remote-as 200
To verify that BGP peers are running, use the show ip bgp neighbors privileged EXEC command. This
is the output of this command on Router A:
Switch# show ip bgp neighbors
BGP neighbor is 129.213.1.1, remote AS 200, external link
BGP version 4, remote router ID 175.220.212.1
BGP state = established, table version = 3, up for 0:10:59
Last read 0:00:29, hold time is 180, keepalive interval is 60 seconds
Minimum time between advertisement runs is 30 seconds
Received 2828 messages, 0 notifications, 0 in queue
Sent 2826 messages, 0 notifications, 0 in queue
Connections established 11; dropped 10
Anything other than state = established means that the peers are not running. The remote router ID is
the highest IP address on that router (or the highest loopback interface). Each time the table is updated
with new information, the table version number increments. A table version number that continually
increments means that a route is flapping, causing continual routing updates.
For exterior protocols, a reference to an IP network from the network router configuration command
controls only which networks are advertised. This is in contrast to Interior Gateway Protocols (IGPs),
such as EIGRP, which also use the network command to specify where to send updates.
For detailed descriptions of BGP configuration, see the “IP Routing Protocols” part of the Cisco IOS IP
Configuration Guide, Release 12.2. For details about specific commands, see the Cisco IOS IP
Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2. See Appendix C, “Unsupported
Commands in Cisco IOS Release 12.2(55)SE,” for a list of BGP commands that are visible but not
supported by the switch.
Managing Routing Policy Changes
Routing policies for a peer include all the configurations that might affect inbound or outbound routing
table updates. When you have defined two routers as BGP neighbors, they form a BGP connection and
exchange routing information. If you later change a BGP filter, weight, distance, version, or timer, or
make a similar configuration change, you must reset the BGP sessions so that the configuration changes
take effect.
There are two types of reset, hard reset and soft reset. Cisco IOS Releases 12.1 and later support a soft
reset without any prior configuration. To use a soft reset without preconfiguration, both BGP peers must
support the soft route refresh capability, which is advertised in the OPEN message sent when the peers
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establish a TCP session. A soft reset allows the dynamic exchange of route refresh requests and routing
information between BGP routers and the subsequent re-advertisement of the respective outbound
routing table.
•
When soft reset generates inbound updates from a neighbor, it is called dynamic inbound soft reset.
•
When soft reset sends a set of updates to a neighbor, it is called outbound soft reset.
A soft inbound reset causes the new inbound policy to take effect. A soft outbound reset causes the new
local outbound policy to take effect without resetting the BGP session. As a new set of updates is sent
during outbound policy reset, a new inbound policy can also take effect.
Table 41-10 lists the advantages and disadvantages hard reset and soft reset.
Table 41-10
Advantages and Disadvantages of Hard and Soft Resets
Type of Reset
Advantages
Disadvantages
Hard reset
No memory overhead
The prefixes in the BGP, IP, and FIB tables
provided by the neighbor are lost. Not
recommended.
Outbound soft reset
No configuration, no storing of routing table
updates
Does not reset inbound routing table updates.
Dynamic inbound soft reset Does not clear the BGP session and cache
Both BGP routers must support the route
Does not require storing of routing table updates refresh capability (in Cisco IOS Release 12.1
and later).
and has no memory overhead
Beginning in privileged EXEC mode, follow these steps to learn if a BGP peer supports the route refresh
capability and to reset the BGP session:
Step 1
Command
Purpose
show ip bgp neighbors
Display whether a neighbor supports the route refresh capability. When supported,
this message appears for the router:
Received route refresh capability from peer.
Step 2
Step 3
Step 4
clear ip bgp {* | address |
peer-group-name}
clear ip bgp {* | address |
peer-group-name} soft out
show ip bgp
show ip bgp neighbors
Reset the routing table on the specified connection.
•
Enter an asterisk (*) to specify that all connections be reset.
•
Enter an IP address to specify the connection to be reset.
•
Enter a peer group name to reset the peer group.
(Optional) Perform an outbound soft reset to reset the inbound routing table on the
specified connection. Use this command if route refresh is supported.
•
Enter an asterisk (*) to specify that all connections be reset.
•
Enter an IP address to specify the connection to be reset.
•
Enter a peer group name to reset the peer group.
Verify the reset by checking information about the routing table and about BGP
neighbors.
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Configuring BGP Decision Attributes
When a BGP speaker receives updates from multiple autonomous systems that describe different paths
to the same destination, it must choose the single best path for reaching that destination. When chosen,
the selected path is entered into the BGP routing table and propagated to its neighbors. The decision is
based on the value of attributes that the update contains and other BGP-configurable factors.
When a BGP peer learns two EBGP paths for a prefix from a neighboring AS, it chooses the best path
and inserts that path in the IP routing table. If BGP multipath support is enabled and the EBGP paths are
learned from the same neighboring autonomous systems, instead of a single best path, multiple paths are
installed in the IP routing table. Then, during packet switching, per-packet or per-destination
load-balancing is performed among the multiple paths. The maximum-paths router configuration
command controls the number of paths allowed.
These factors summarize the order in which BGP evaluates the attributes for choosing the best path:
1.
If the path specifies a next hop that is inaccessible, drop the update. The BGP next-hop attribute,
automatically determined by the software, is the IP address of the next hop that is going to be used
to reach a destination. For EBGP, this is usually the IP address of the neighbor specified by the
neighbor remote-as router configuration command. You can disable next-hop processing by using
route maps or the neighbor next-hop-self router configuration command.
2.
Prefer the path with the largest weight (a Cisco proprietary parameter). The weight attribute is local
to the router and not propagated in routing updates. By default, the weight attribute is 32768 for
paths that the router originates and zero for other paths. Routes with the largest weight are preferred.
You can use access lists, route maps, or the neighbor weight router configuration command to set
weights.
3.
Prefer the route with the highest local preference. Local preference is part of the routing update and
exchanged among routers in the same AS. The default value of the local preference attribute is 100.
You can set local preference by using the bgp default local-preference router configuration
command or by using a route map.
4.
Prefer the route that was originated by BGP running on the local router.
5.
Prefer the route with the shortest AS path.
6.
Prefer the route with the lowest origin type. An interior route or IGP is lower than a route learned
by EGP, and an EGP-learned route is lower than one of unknown origin or learned in another way.
7.
Prefer the route with the lowest multi -exit discriminator (MED) metric attribute if the neighboring
AS is the same for all routes considered. You can configure the MED by using route maps or by
using the default-metric router configuration command. When an update is sent to an IBGP peer,
the MED is included.
8.
Prefer the external (EBGP) path over the internal (IBGP) path.
9.
Prefer the route that can be reached through the closest IGP neighbor (the lowest IGP metric). This
means that the router will prefer the shortest internal path within the AS to reach the destination (the
shortest path to the BGP next-hop).
10. If the following conditions are all true, insert the route for this path into the IP routing table:
•
Both the best route and this route are external.
•
Both the best route and this route are from the same neighboring autonomous system.
•
maximum-paths is enabled.
11. If multipath is not enabled, prefer the route with the lowest IP address value for the BGP router ID.
The router ID is usually the highest IP address on the router or the loopback (virtual) address, but
might be implementation-specific.
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Beginning in privileged EXEC mode, follow these steps to configure some decision attributes:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router bgp autonomous-system
Enable a BGP routing process, assign it an AS number,
and enter router configuration mode.
Step 3
bgp best-path as-path ignore
(Optional) Configure the router to ignore AS path length
in selecting a route.
Step 4
neighbor {ip-address | peer-group-name} next-hop-self (Optional) Disable next-hop processing on BGP updates
to a neighbor by entering a specific IP address to be used
instead of the next-hop address.
Step 5
neighbor {ip-address | peer-group-name} weight
weight
(Optional) Assign a weight to a neighbor connection.
Acceptable values are from 0 to 65535; the largest weight
is the preferred route. Routes learned through another
BGP peer have a default weight of 0; routes sourced by the
local router have a default weight of 32768.
Step 6
default-metric number
(Optional) Set a MED metric to set preferred paths to
external neighbors. All routes without a MED will also be
set to this value. The range is 1 to 4294967295. The lowest
value is the most desirable.
Step 7
bgp bestpath med missing-as-worst
(Optional) Configure the switch to consider a missing
MED as having a value of infinity, making the path
without a MED value the least desirable path.
Step 8
bgp always-compare med
(Optional) Configure the switch to compare MEDs for
paths from neighbors in different autonomous systems. By
default, MED comparison is only done among paths in the
same AS.
Step 9
bgp bestpath med confed
(Optional) Configure the switch to consider the MED in
choosing a path from among those advertised by different
subautonomous systems within a confederation.
Step 10
bgp deterministic med
(Optional) Configure the switch to consider the MED
variable when choosing among routes advertised by
different peers in the same AS.
Step 11
bgp default local-preference value
(Optional) Change the default local preference value. The
range is 0 to 4294967295; the default value is 100. The
highest local preference value is preferred.
Step 12
maximum-paths number
(Optional) Configure the number of paths to be added to
the IP routing table. The default is to only enter the best
path in the routing table. The range is from 1 to 16. Having
multiple paths allows load-balancing among the paths.
(Although the switch software allows a maximum of
32 equal-cost routes, the switch hardware will never use
more than 16 paths per route.)
Step 13
end
Return to privileged EXEC mode.
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Command
Purpose
Step 14
show ip bgp
show ip bgp neighbors
Verify the reset by checking information about the routing
table and about BGP neighbors.
Step 15
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no form of each command to return to the default state.
Configuring BGP Filtering with Route Maps
Within BGP, route maps can be used to control and to modify routing information and to define the
conditions by which routes are redistributed between routing domains. See the “Using Route Maps to
Redistribute Routing Information” section on page 41-94 for more information about route maps. Each
route map has a name that identifies the route map (map tag) and an optional sequence number.
Beginning in privileged EXEC mode, follow these steps to use a route map to disable next-hop
processing:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
route-map map-tag [[permit | deny] |
sequence-number]]
Create a route map, and enter route-map configuration mode.
Step 3
set ip next-hop ip-address [...ip-address]
[peer-address]
(Optional) Set a route map to disable next-hop processing
•
In an inbound route map, set the next hop of matching routes to
be the neighbor peering address, overriding third-party next hops.
•
In an outbound route map of a BGP peer, set the next hop to the
peering address of the local router, disabling the next-hop
calculation.
Step 4
end
Return to privileged EXEC mode.
Step 5
show route-map [map-name]
Display all route maps configured or only the one specified to verify
configuration.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no route-map map-tag command to delete the route map. Use the no set ip next-hop ip-address
command to re-enable next-hop processing.
Configuring BGP Filtering by Neighbor
You can filter BGP advertisements by using AS-path filters, such as the as-path access-list global
configuration command and the neighbor filter-list router configuration command. You can also use
access lists with the neighbor distribute-list router configuration command. Distribute-list filters are
applied to network numbers. See the “Controlling Advertising and Processing in Routing Updates”
section on page 41-102 for information about the distribute-list command.
You can use route maps on a per-neighbor basis to filter updates and to modify various attributes. A route
map can be applied to either inbound or outbound updates. Only the routes that pass the route map are
sent or accepted in updates. On both inbound and outbound updates, matching is supported based on AS
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path, community, and network numbers. Autonomous system path matching requires the match as-path
access-list route-map command, community based matching requires the match community-list
route-map command, and network-based matching requires the ip access-list global configuration
command.
Beginning in privileged EXEC mode, follow these steps to apply a per-neighbor route map:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router bgp autonomous-system
Enable a BGP routing process, assign it an AS number, and enter
router configuration mode.
Step 3
neighbor {ip-address | peer-group name}
distribute-list {access-list-number | name}
{in | out}
(Optional) Filter BGP routing updates to or from neighbors as
specified in an access list.
Step 4
neighbor {ip-address | peer-group name}
route-map map-tag {in | out}
(Optional) Apply a route map to filter an incoming or outgoing
route.
Step 5
end
Return to privileged EXEC mode.
Step 6
show ip bgp neighbors
Verify the configuration.
Step 7
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Note
You can also use the neighbor prefix-list router
configuration command to filter updates, but you cannot use
both commands to configure the same BGP peer.
Use the no neighbor distribute-list command to remove the access list from the neighbor. Use the no
neighbor route-map map-tag router configuration command to remove the route map from the
neighbor.
Another method of filtering is to specify an access list filter on both incoming and outbound updates,
based on the BGP autonomous system paths. Each filter is an access list based on regular expressions.
(See the “Regular Expressions” appendix in the Cisco IOS Dial Technologies Command Reference,
Release 12.2 for more information on forming regular expressions.) To use this method, define an
autonomous system path access list, and apply it to updates to and from particular neighbors.
Beginning in privileged EXEC mode, follow these steps to configure BGP path filtering:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip as-path access-list access-list-number
{permit | deny} as-regular-expressions
Define a BGP-related access list.
Step 3
router bgp autonomous-system
Enter BGP router configuration mode.
Step 4
neighbor {ip-address | peer-group name}
filter-list {access-list-number | name} {in |
out | weight weight}
Establish a BGP filter based on an access list.
Step 5
end
Return to privileged EXEC mode.
Step 6
show ip bgp neighbors [paths
regular-expression]
Verify the configuration.
Step 7
copy running-config startup-config
(Optional) Save your entries in the configuration file.
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Configuring Prefix Lists for BGP Filtering
You can use prefix lists as an alternative to access lists in many BGP route filtering commands, including
the neighbor distribute-list router configuration command. The advantages of using prefix lists include
performance improvements in loading and lookup of large lists, incremental update support, easier CLI
configuration, and greater flexibility.
Filtering by a prefix list involves matching the prefixes of routes with those listed in the prefix list, as
when matching access lists. When there is a match, the route is used. Whether a prefix is permitted or
denied is based upon these rules:
•
An empty prefix list permits all prefixes.
•
An implicit deny is assumed if a given prefix does not match any entries in a prefix list.
•
When multiple entries of a prefix list match a given prefix, the sequence number of a prefix list entry
identifies the entry with the lowest sequence number.
By default, sequence numbers are generated automatically and incremented in units of five. If you
disable the automatic generation of sequence numbers, you must specify the sequence number for each
entry. You can specify sequence values in any increment. If you specify increments of one, you cannot
insert additional entries into the list; if you choose very large increments, you might run out of values.
You do not need to specify a sequence number when removing a configuration entry. Show commands
include the sequence numbers in their output.
Before using a prefix list in a command, you must set up the prefix list. Beginning in privileged EXEC
mode, follow these steps to create a prefix list or to add an entry to a prefix list:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip prefix-list list-name [seq seq-value] deny | Create a prefix list with an optional sequence number to deny or
permit network/len [ge ge-value] [le le-value] permit access for matching conditions. You must enter at least one
permit or deny clause.
•
network/len is the network number and length (in bits) of the
network mask.
•
(Optional) ge and le values specify the range of the prefix length
to be matched.The specified ge-value and le-value must satisfy
this condition: len < ge-value < le-value < 32
Step 3
ip prefix-list list-name seq seq-value deny | (Optional) Add an entry to a prefix list, and assign a sequence
permit network/len [ge ge-value] [le le-value] number to the entry.
Step 4
end
Return to privileged EXEC mode.
Step 5
show ip prefix list [detail | summary] name
[network/len] [seq seq-num] [longer]
[first-match]
Verify the configuration by displaying information about a prefix
list or prefix list entries.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To delete a prefix list and all of its entries, use the no ip prefix-list list-name global configuration
command. To delete an entry from a prefix list, use the no ip prefix-list seq seq-value global
configuration command. To disable automatic generation of sequence numbers, use the no ip prefix-list
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sequence number command; to reenable automatic generation, use the ip prefix-list sequence number
command. To clear the hit-count table of prefix list entries, use the clear ip prefix-list privileged EXEC
command.
Configuring BGP Community Filtering
One way that BGP controls the distribution of routing information based on the value of the
COMMUNITIES attribute. The attribute is a way to groups destinations into communities and to apply
routing decisions based on the communities. This method simplifies configuration of a BGP speaker to
control distribution of routing information.
A community is a group of destinations that share some common attribute. Each destination can belong
to multiple communities. AS administrators can define to which communities a destination belongs. By
default, all destinations belong to the general Internet community. The community is identified by the
COMMUNITIES attribute, an optional, transitive, global attribute in the numerical range from
1 to 4294967200. These are some predefined, well-known communities:
•
internet—Advertise this route to the Internet community. All routers belong to it.
•
no-export—Do not advertise this route to EBGP peers.
•
no-advertise—Do not advertise this route to any peer (internal or external).
•
local-as—Do not advertise this route to peers outside the local autonomous system.
Based on the community, you can control which routing information to accept, prefer, or distribute to
other neighbors. A BGP speaker can set, append, or modify the community of a route when learning,
advertising, or redistributing routes. When routes are aggregated, the resulting aggregate has a
COMMUNITIES attribute that contains all communities from all the initial routes.
You can use community lists to create groups of communities to use in a match clause of a route map.
As with an access list, a series of community lists can be created. Statements are checked until a match
is found. As soon as one statement is satisfied, the test is concluded.
To set the COMMUNITIES attribute and match clauses based on communities, see the match
community-list and set community route-map configuration commands in the “Using Route Maps to
Redistribute Routing Information” section on page 41-94.
By default, no COMMUNITIES attribute is sent to a neighbor. You can specify that the COMMUNITIES
attribute be sent to the neighbor at an IP address by using the neighbor send-community router
configuration command.
Beginning in privileged EXEC mode, follow these steps to create and to apply a community list:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip community-list community-list-number Create a community list, and assign it a number.
{permit | deny} community-number
• The community-list-number is an integer from 1 to 99 that
identifies one or more permit or deny groups of communities.
•
The community-number is the number configured by a set
community route-map configuration command.
Step 3
router bgp autonomous-system
Enter BGP router configuration mode.
Step 4
neighbor {ip-address | peer-group name}
send-community
Specify that the COMMUNITIES attribute be sent to the neighbor at
this IP address.
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Command
Purpose
Step 5
set comm-list list-num delete
(Optional) Remove communities from the community attribute of an
inbound or outbound update that match a standard or extended
community list specified by a route map.
Step 6
exit
Return to global configuration mode.
Step 7
ip bgp-community new-format
(Optional) Display and parse BGP communities in the format AA:NN.
A BGP community is displayed in a two-part format 2 bytes long. The
Cisco default community format is in the format NNAA. In the most
recent RFC for BGP, a community takes the form AA:NN, where the
first part is the AS number and the second part is a 2-byte number.
Step 8
Step 9
Step 10
end
Return to privileged EXEC mode.
show ip bgp community
Verify the configuration.
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Configuring BGP Neighbors and Peer Groups
Often many BGP neighbors are configured with the same update policies (that is, the same outbound
route maps, distribute lists, filter lists, update source, and so on). Neighbors with the same update
policies can be grouped into peer groups to simplify configuration and to make updating more efficient.
When you have configured many peers, we recommend this approach.
To configure a BGP peer group, you create the peer group, assign options to the peer group, and add
neighbors as peer group members. You configure the peer group by using the neighbor router
configuration commands. By default, peer group members inherit all the configuration options of the
peer group, including the remote-as (if configured), version, update-source, out-route-map,
out-filter-list, out-dist-list, minimum-advertisement-interval, and next-hop-self. All peer group members
also inherit changes made to the peer group. Members can also be configured to override the options that
do not affect outbound updates.
To assign configuration options to an individual neighbor, specify any of these router configuration
commands by using the neighbor IP address. To assign the options to a peer group, specify any of the
commands by using the peer group name. You can disable a BGP peer or peer group without removing
all the configuration information by using the neighbor shutdown router configuration command.
Beginning in privileged EXEC mode, use these commands to configure BGP peers:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router bgp autonomous-system
Enter BGP router configuration mode.
Step 3
neighbor peer-group-name peer-group
Create a BGP peer group.
Step 4
neighbor ip-address peer-group
peer-group-name
Make a BGP neighbor a member of the peer group.
Step 5
neighbor {ip-address | peer-group-name}
remote-as number
Specify a BGP neighbor. If a peer group is not configured with a
remote-as number, use this command to create peer groups
containing EBGP neighbors. The range is 1 to 65535.
Step 6
neighbor {ip-address | peer-group-name}
description text
(Optional) Associate a description with a neighbor.
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Command
Purpose
Step 7
neighbor {ip-address | peer-group-name}
default-originate [route-map map-name]
(Optional) Allow a BGP speaker (the local router) to send the
default route 0.0.0.0 to a neighbor for use as a default route.
Step 8
neighbor {ip-address | peer-group-name}
send-community
(Optional) Specify that the COMMUNITIES attribute be sent to
the neighbor at this IP address.
Step 9
neighbor {ip-address | peer-group-name}
update-source interface
(Optional) Allow internal BGP sessions to use any operational
interface for TCP connections.
Step 10
neighbor {ip-address | peer-group-name}
ebgp-multihop
(Optional) Allow BGP sessions, even when the neighbor is not
on a directly connected segment. The multihop session is not
established if the only route to the multihop peer’s address is the
default route (0.0.0.0).
Step 11
neighbor {ip-address | peer-group-name}
local-as number
(Optional) Specify an AS number to use as the local AS. The
range is 1 to 65535.
Step 12
neighbor {ip-address | peer-group-name}
advertisement-interval seconds
(Optional) Set the minimum interval between sending BGP
routing updates.
Step 13
neighbor {ip-address | peer-group-name}
maximum-prefix maximum [threshold]
(Optional) Control how many prefixes can be received from a
neighbor. The range is 1 to 4294967295. The threshold
(optional) is the percentage of maximum at which a warning
message is generated. The default is 75 percent.
Step 14
neighbor {ip-address | peer-group-name}
next-hop-self
(Optional) Disable next-hop processing on the BGP updates to a
neighbor.
Step 15
neighbor {ip-address | peer-group-name}
password string
(Optional) Set MD5 authentication on a TCP connection to a
BGP peer. The same password must be configured on both BGP
peers, or the connection between them is not made.
Step 16
neighbor {ip-address | peer-group-name}
route-map map-name {in | out}
(Optional) Apply a route map to incoming or outgoing routes.
Step 17
neighbor {ip-address | peer-group-name}
send-community
(Optional) Specify that the COMMUNITIES attribute be sent to
the neighbor at this IP address.
Step 18
neighbor {ip-address | peer-group-name} timers (Optional) Set timers for the neighbor or peer group.
keepalive holdtime
• The keepalive interval is the time within which keepalive
messages are sent to peers. The range is 1 to 4294967295
seconds; the default is 60.
•
The holdtime is the interval after which a peer is declared
inactive after not receiving a keepalive message from it. The
range is 1 to 4294967295 seconds; the default is 180.
Step 19
neighbor {ip-address | peer-group-name} weight (Optional) Specify a weight for all routes from a neighbor.
weight
Step 20
neighbor {ip-address | peer-group-name}
distribute-list {access-list-number | name} {in |
out}
(Optional) Filter BGP routing updates to or from neighbors, as
specified in an access list.
Step 21
neighbor {ip-address | peer-group-name}
filter-list access-list-number {in | out | weight
weight}
(Optional) Establish a BGP filter.
Step 22
neighbor {ip-address | peer-group-name}
version value
(Optional) Specify the BGP version to use when communicating
with a neighbor.
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Command
Purpose
Step 23
neighbor {ip-address | peer-group-name}
soft-reconfiguration inbound
(Optional) Configure the software to start storing received
updates.
Step 24
end
Return to privileged EXEC mode.
Step 25
show ip bgp neighbors
Verify the configuration.
Step 26
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To disable an existing BGP neighbor or neighbor peer group, use the neighbor shutdown router
configuration command. To enable a previously existing neighbor or neighbor peer group that had been
disabled, use the no neighbor shutdown router configuration command.
Configuring Aggregate Addresses
Classless interdomain routing (CIDR) enables you to create aggregate routes (or supernets) to minimize
the size of routing tables. You can configure aggregate routes in BGP either by redistributing an
aggregate route into BGP or by creating an aggregate entry in the BGP routing table. An aggregate
address is added to the BGP table when there is at least one more specific entry in the BGP table.
Beginning in privileged EXEC mode, use these commands to create an aggregate address in the routing
table:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router bgp autonomous-system
Enter BGP router configuration mode.
Step 3
aggregate-address address mask
Create an aggregate entry in the BGP routing table. The aggregate
route is advertised as coming from the AS, and the atomic aggregate
attribute is set to indicate that information might be missing.
Step 4
aggregate-address address mask as-set
(Optional) Generate AS set path information. This command
creates an aggregate entry following the same rules as the previous
command, but the advertised path will be an AS_SET consisting of
all elements contained in all paths. Do not use this keyword when
aggregating many paths because this route must be continually
withdrawn and updated.
Step 5
aggregate-address address-mask
summary-only
(Optional) Advertise summary addresses only.
Step 6
aggregate-address address mask
suppress-map map-name
(Optional) Suppress selected, more specific routes.
Step 7
aggregate-address address mask
advertise-map map-name
(Optional) Generate an aggregate based on conditions specified by
the route map.
Step 8
aggregate-address address mask
attribute-map map-name
(Optional) Generate an aggregate with attributes specified in the
route map.
Step 9
end
Return to privileged EXEC mode.
Step 10
show ip bgp neighbors [advertised-routes]
Verify the configuration.
Step 11
copy running-config startup-config
(Optional) Save your entries in the configuration file.
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Configuring BGP
To delete an aggregate entry, use the no aggregate-address address mask router configuration
command. To return options to the default values, use the command with keywords.
Configuring Routing Domain Confederations
One way to reduce the IBGP mesh is to divide an autonomous system into multiple subautonomous
systems and to group them into a single confederation that appears as a single autonomous system. Each
autonomous system is fully meshed within itself and has a few connections to other autonomous systems
in the same confederation. Even though the peers in different autonomous systems have EBGP sessions,
they exchange routing information as if they were IBGP peers. Specifically, the next hop, MED, and
local preference information is preserved. You can then use a single IGP for all of the autonomous
systems.
To configure a BGP confederation, you must specify a confederation identifier that acts as the
autonomous system number for the group of autonomous systems.
Beginning in privileged EXEC mode, use these commands to configure a BGP confederation:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router bgp autonomous-system
Enter BGP router configuration mode.
Step 3
bgp confederation identifier autonomous-system Configure a BGP confederation identifier.
Step 4
bgp confederation peers autonomous-system
[autonomous-system ...]
Specify the autonomous systems that belong to the
confederation and that will be treated as special EBGP peers.
Step 5
end
Return to privileged EXEC mode.
Step 6
show ip bgp neighbor
Verify the configuration.
show ip bgp network
Step 7
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Configuring BGP Route Reflectors
BGP requires that all of the IBGP speakers be fully meshed. When a router receives a route from an
external neighbor, it must advertise it to all internal neighbors. To prevent a routing information loop,
all IBPG speakers must be connected. The internal neighbors do not send routes learned from internal
neighbors to other internal neighbors.
With route reflectors, all IBGP speakers need not be fully meshed because another method is used to
pass learned routes to neighbors. When you configure an internal BGP peer to be a route reflector, it is
responsible for passing IBGP learned routes to a set of IBGP neighbors. The internal peers of the route
reflector are divided into two groups: client peers and nonclient peers (all the other routers in the
autonomous system). A route reflector reflects routes between these two groups. The route reflector and
its client peers form a cluster. The nonclient peers must be fully meshed with each other, but the client
peers need not be fully meshed. The clients in the cluster do not communicate with IBGP speakers
outside their cluster.
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When the route reflector receives an advertised route, it takes one of these actions, depending on the
neighbor:
•
A route from an external BGP speaker is advertised to all clients and nonclient peers.
•
A route from a nonclient peer is advertised to all clients.
•
A route from a client is advertised to all clients and nonclient peers. Hence, the clients need not be
fully meshed.
Usually a cluster of clients have a single route reflector, and the cluster is identified by the route reflector
router ID. To increase redundancy and to avoid a single point of failure, a cluster might have more than
one route reflector. In this case, all route reflectors in the cluster must be configured with the same 4-byte
cluster ID so that a route reflector can recognize updates from route reflectors in the same cluster. All
the route reflectors serving a cluster should be fully meshed and should have identical sets of client and
nonclient peers.
Beginning in privileged EXEC mode, use these commands to configure a route reflector and clients:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router bgp autonomous-system
Enter BGP router configuration mode.
Step 3
neighbor ip-address | peer-group-name
route-reflector-client
Configure the local router as a BGP route reflector and the
specified neighbor as a client.
Step 4
bgp cluster-id cluster-id
(Optional) Configure the cluster ID if the cluster has more than
one route reflector.
Step 5
no bgp client-to-client reflection
(Optional) Disable client-to-client route reflection. By default,
the routes from a route reflector client are reflected to other
clients. However, if the clients are fully meshed, the route
reflector does not need to reflect routes to clients.
Step 6
end
Return to privileged EXEC mode.
Step 7
show ip bgp
Verify the configuration. Display the originator ID and the
cluster-list attributes.
Step 8
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Configuring Route Dampening
Route flap dampening is a BGP feature designed to minimize the propagation of flapping routes across
an internetwork. A route is considered to be flapping when it is repeatedly available, then unavailable,
then available, then unavailable, and so on. When route dampening is enabled, a numeric penalty value
is assigned to a route when it flaps. When a route’s accumulated penalties reach a configurable limit,
BGP suppresses advertisements of the route, even if the route is running. The reuse limit is a
configurable value that is compared with the penalty. If the penalty is less than the reuse limit, a
suppressed route that is up is advertised again.
Dampening is not applied to routes that are learned by IBGP. This policy prevents the IBGP peers from
having a higher penalty for routes external to the AS.
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Beginning in privileged EXEC mode, use these commands to configure BGP route dampening:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router bgp autonomous-system
Enter BGP router configuration mode.
Step 3
bgp dampening
Enable BGP route dampening.
Step 4
bgp dampening half-life reuse suppress
max-suppress [route-map map]
(Optional) Change the default values of route dampening
factors.
Step 5
end
Return to privileged EXEC mode.
Step 6
show ip bgp flap-statistics [{regexp regexp} |
(Optional) Monitor the flaps of all paths that are flapping. The
{filter-list list} | {address mask [longer-prefix]}] statistics are deleted when the route is not suppressed and is
stable.
Step 7
show ip bgp dampened-paths
Step 8
clear ip bgp flap-statistics [{regexp regexp} |
(Optional) Clear BGP flap statistics to make it less likely that a
{filter-list list} | {address mask [longer-prefix]} route will be dampened.
Step 9
clear ip bgp dampening
(Optional) Clear route dampening information, and unsuppress
the suppressed routes.
Step 10
copy running-config startup-config
(Optional) Save your entries in the configuration file.
(Optional) Display the dampened routes, including the time
remaining before they are suppressed.
To disable flap dampening, use the no bgp dampening router configuration command without
keywords. To set dampening factors back to the default values, use the no bgp dampening router
configuration command with values.
Monitoring and Maintaining BGP
You can remove all contents of a particular cache, table, or database. This might be necessary when the
contents of the particular structure have become or are suspected to be invalid.
You can display specific statistics, such as the contents of BGP routing tables, caches, and databases.
You can use the information to get resource utilization and solve network problems. You can also display
information about node reachability and discover the routing path your device’s packets are taking
through the network.
Table 41-8 lists the privileged EXEC commands for clearing and displaying BGP. For explanations of
the display fields, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols,
Release 12.2.
Table 41-11
IP BGP Clear and Show Commands
Command
Purpose
clear ip bgp address
Reset a particular BGP connection.
clear ip bgp *
Reset all BGP connections.
clear ip bgp peer-group tag
Remove all members of a BGP peer group.
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Table 41-11
IP BGP Clear and Show Commands (continued)
Command
Purpose
show ip bgp prefix
Display peer groups and peers not in peer groups to which the
prefix has been advertised. Also display prefix attributes such as
the next hop and the local prefix.
show ip bgp cidr-only
Display all BGP routes that contain subnet and supernet network
masks.
show ip bgp community [community-number] [exact]
Display routes that belong to the specified communities.
show ip bgp community-list community-list-number
[exact-match]
Display routes that are permitted by the community list.
show ip bgp filter-list access-list-number
Display routes that are matched by the specified AS path access
list.
show ip bgp inconsistent-as
Display the routes with inconsistent originating autonomous
systems.
show ip bgp regexp regular-expression
Display the routes that have an AS path that matches the specified
regular expression entered on the command line.
show ip bgp
Display the contents of the BGP routing table.
show ip bgp neighbors [address]
Display detailed information on the BGP and TCP connections to
individual neighbors.
show ip bgp neighbors [address] [advertised-routes |
dampened-routes | flap-statistics | paths
regular-expression | received-routes | routes]
Display routes learned from a particular BGP neighbor.
show ip bgp paths
Display all BGP paths in the database.
show ip bgp peer-group [tag] [summary]
Display information about BGP peer groups.
show ip bgp summary
Display the status of all BGP connections.
You can also enable the logging of messages generated when a BGP neighbor resets, comes up, or goes
down by using the bgp log-neighbor changes router configuration command.
Configuring ISO CLNS Routing
The International Organization for Standardization (ISO) Connectionless Network Service (CLNS)
protocol is a standard for the network layer of the Open System Interconnection (OSI) model. Addresses
in the ISO network architecture are referred to as network service access point (NSAP) addresses and
network entity titles (NETs). Each node in an OSI network has one or more NETs. In addition, each node
has many NSAP addresses.
When you enable connectionless routing on the switch by using the clns routing global configuration
command, the switch makes only forwarding decisions, with no routing-related functionality. For
dynamic routing, you must also enable a routing protocol. The switch supports the Intermediate
System-to-Intermediate System (IS-IS) dynamic routing protocol that is based on the OSI routing
protocol for ISO CLNS networks.
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When dynamically routing, you use IS-IS. This routing protocol supports the concept of areas. Within
an area, all routers know how to reach all the system IDs. Between areas, routers know how to reach the
proper area. IS-IS supports two levels of routing: station routing (within an area) and area routing
(between areas).
The key difference between the ISO IGRP and IS-IS NSAP addressing schemes is in the definition of
area addresses. Both use the system ID for Level 1 routing (routing within an area). However, they differ
in the way addresses are specified for area routing. An ISO IGRP NSAP address includes three separate
fields for routing: the domain, area, and system ID. An IS-IS address includes two fields: a single
continuous area field (comprising the domain and area fields) and the system ID.
Note
For more detailed information about ISO CLNS, see the Cisco IOS Apollo Domain, Banyan VINES,
DECnet, ISO CLNS and XNS Configuration Guide, Release 12.2. For complete syntax and usage
information for the commands used in this chapter, see the Cisco IOS Apollo Domain, Banyan VINES,
DECnet, ISO CLNS and XNS Command Reference, Release 12.2, use the IOS command reference master
index, or search online.
Configuring IS-IS Dynamic Routing
IS-IS is an ISO dynamic routing protocol (described in ISO 105890). Unlike other routing protocols,
enabling IS-IS requires that you create an IS-IS routing process and assign it to a specific interface,
rather than to a network. You can specify more than one IS-IS routing process per Layer 3 switch or
router by using the multiarea IS-IS configuration syntax. You then configure the parameters for each
instance of the IS-IS routing process.
Small IS-IS networks are built as a single area that includes all the routers in the network. As the network
grows larger, it is usually reorganized into a backbone area made up of the connected set of all Level 2
routers from all areas, which is in turn connected to local areas. Within a local area, routers know how
to reach all system IDs. Between areas, routers know how to reach the backbone, and the backbone
routers know how to reach other areas.
Routers establish Level 1 adjacencies to perform routing within a local area (station routing). Routers
establish Level 2 adjacencies to perform routing between Level 1 areas (area routing).
A single Cisco router can participate in routing in up to 29 areas and can perform Level 2 routing in the
backbone. In general, each routing process corresponds to an area. By default, the first instance of the
routing process configured performs both Level 1and Level 2 routing. You can configure additional
router instances, which are automatically treated as Level 1 areas. You must configure the parameters
for each instance of the IS-IS routing process individually.
For IS-IS multiarea routing, you can configure only one process to perform Level 2 routing, although
you can define up to 29 Level 1 areas for each Cisco unit. If Level 2 routing is configured on any process,
all additional processes are automatically configured as Level 1. You can configure this process to
perform Level 1 routing at the same time. If Level 2 routing is not desired for a router instance, remove
the Level 2 capability using the is-type global configuration command. Use the is-type command also
to configure a different router instance as a Level 2 router.
Note
For more detailed information about IS-IS, see the “IP Routing Protocols” chapter of the Cisco IOS IP
Configuration Guide, Release 12.2. For complete syntax and usage information for the commands used
in this section, see the Cisco IOS IP Command Reference, Release 12.2.
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These sections briefly describes how to configure IS-IS routing.
•
Default IS-IS Configuration, page 41-67
•
Enabling IS-IS Routing, page 41-68
•
Configuring IS-IS Global Parameters, page 41-70
•
Configuring IS-IS Interface Parameters, page 41-72
Default IS-IS Configuration
Table 41-12
Default IS-IS Configuration
Feature
Default Setting
Ignore link-state PDU (LSP) errors
Enabled.
IS-IS type
Conventional IS-IS: the router acts as both a Level 1 (station) and a Level 2
(area) router.
Multiarea IS-IS: the first instance of the IS-IS routing process is a Level 1-2
router. Remaining instances are Level 1 routers.
Default-information originate
Disabled.
Log IS-IS adjacency state changes.
Disabled.
LSP generation throttling timers
Maximum interval between two consecutive occurrences: 5 seconds.
Initial LSP generation delay: 50 ms.
Hold time between the first and second LSP generation: 5000 ms.
LSP maximum lifetime (without a refresh)
1200 seconds (20 minutes) before t.he LSP packet is deleted.
LSP refresh interval
Send LSP refreshes every 900 seconds (15 minutes).
Maximum LSP packet size
1497 bytes.
NSF Awareness
1
Partial route computation (PRC) throttling
timers
Enabled2 . Allows Layer 3 switches to continue forwarding packets from a
neighboring NSF-capable router during hardware or software changes.
Maximum PRC wait interval: 5 seconds.
Initial PRC calculation delay after a topology change: 2000 ms.
Hold time between the first and second PRC calculation: 5000 ms.
Partition avoidance
Disabled.
Password
No area or domain password is defined, and authentication is disabled.
Set-overload-bit
Disabled. When enabled, if no arguments are entered, the overload bit is set
immediately and remains set until you enter the no set-overload-bit command.
Shortest path first (SPF) throttling timers
Maximum interval between consecutive SFPs: 10 seconds.
Initial SFP calculation after a topology change: 5500 ms.
Holdtime between the first and second SFP calculation: 5500 ms.
Summary-address
Disabled.
1. NSF = Nonstop Forwarding
2. IS-IS NSF awareness is enabled for IPv4 on switches running Cisco IOS Release 12.2(25)SEG or later.
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Nonstop Forwarding Awareness
The integrated IS-IS NSF Awareness feature is supported for IPv4, beginning with Cisco IOS
Release 12.2(25)SEG. The feature allows customer premises equipment (CPE) routers that are
NSF-aware to help NSF-capable routers perform nonstop forwarding of packets. The local router is not
necessarily performing NSF, but its awareness of NSF allows the integrity and accuracy of the routing
database and link-state database on the neighboring NSF-capable router to be maintained during the
switchover process.
This feature is automatically enabled and requires no configuration. For more information on this
feature, see the Integrated IS-IS Nonstop Forwarding (NSF) Awareness Feature Guide.
Enabling IS-IS Routing
To enable IS-IS, you specify a name and NET for each routing process. You then enable IS-IS routing
on the interface and specify the area for each instance of the routing process.
Beginning in privileged EXEC mode, follow these steps to enable IS-IS and specify the area for each
instance of the IS-IS routing process:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
clns routing
Enable ISO connectionless routing on the switch.
Step 3
router isis [area tag]
Enable the IS-IS routing for the specified routing process and enter IS-IS
routing configuration mode.
(Optional) Use the area tag argument to identify the area to which the IS-IS
router is assigned. You must enter a value if you are configuring multiple
IS-IS areas.
The first IS-IS instance configured is Level 1-2 by default. Later instances
are automatically Level 1. You can change the level of routing by using the
is-type global configuration command.
Step 4
net network-entity-title
Configure the NETs for the routing process. If you are configuring
multiarea IS-IS, specify a NET for each routing process. You can specify a
name for a NET and for an address.
Step 5
is-type {level-1 | level-1-2 |
level-2-only}
(Optional) You can configure the router to act as a Level 1 (station) router,
a Level 2 (area) router for multi-area routing, or both (the default):
•
level-1—act as a station router only
•
level-1-2—act as both a station router and an area router
•
level 2—act as an area router only
Step 6
exit
Return to global configuration mode.
Step 7
interface interface-id
Specify an interface to route IS-IS, and enter interface configuration mode.
If the interface is not already configured as a Layer 3 interface, enter the no
switchport command to put it into Layer 3 mode.
Step 8
ip router isis [area tag]
Configure an IS-IS routing process for ISO CLNS on the interface and
attach an area designator to the routing process.
Step 9
clns router isis [area tag]
Enable ISO CLNS on the interface.
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Command
Purpose
Step 10
ip address ip-address-mask
Define the IP address for the interface. An IP address is required on all
interfaces in an area enabled for IS-IS if any one interface is configured for
IS-IS routing.
Step 11
end
Return to privileged EXEC mode.
Step 12
show isis [area tag] database detail
Verify your entries.
Step 13
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To disable IS-IS routing, use the no router isis area-tag router configuration command.
This example shows how to configure three routers to run conventional IS-IS as an IP routing protocol.
In conventional IS-IS, all routers act as Level 1 and Level 2 routers (by default).
Router A
Switch(config)# clns routing
Switch(config)# router isis
Switch(config-router)# net 49.0001.0000.0000.000a.00
Switch(config-router)# exit
Switch(config)# interface gigabitethernet1/0/1
Switch(config-if)# ip router isis
Switch(config-if)# clns router isis
Switch(config)# interface gigabitethernet1/0/2
Switch(config-if)# ip router isis
Switch(config-if)# clns router isis
Switch(config-router)# exit
Router B
Switch(config)# clns routing
Switch(config)# router isis
Switch(config-router)# net 49.0001.0000.0000.000b.00
Switch(config-router)# exit
Switch(config)# interface gigabitethernet1/0/1
Switch(config-if)# ip router isis
Switch(config-if)# clns router isis
Switch(config)# interface gigabitethernet1/0/2
Switch(config-if)# ip router isis
Switch(config-if)# clns router isis
Switch(config-router)# exit
Router C
Switch(config)# clns routing
Switch(config)# router isis
Switch(config-router)# net 49.0001.0000.0000.000c.00
Switch(config-router)# exit
Switch(config)# interface gigabitethernet1/0/1
Switch(config-if)# ip router isis
Switch(config-if)# clns router isis
Switch(config)# interface gigabitethernet1/0/2
Switch(config-if)# ip router isis
Switch(config-if)# clns router isis
Switch(config-router)# exit
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Configuring IS-IS Global Parameters
These are some optional IS-IS global parameters that you can configure:
•
You can force a default route into an IS-IS routing domain by configuring a default route controlled
by a route map. You can also specify other filtering options configurable under a route map.
•
You can configure the router to ignore IS-IS LSPs that are received with internal checksum errors
or to purge corrupted LSPs, which causes the initiator of the LSP to regenerate it.
•
You can assign passwords to areas and domains.
•
You can create aggregate addresses that are represented in the routing table by a summary address
(route-summarization). Routes learned from other routing protocols can also be summarized. The
metric used to advertise the summary is the smallest metric of all the specific routes.
•
You can set an overload bit.
•
You can configure the LSP refresh interval and the maximum time that an LSP can remain in the
router database without a refresh
•
You can set the throttling timers for LSP generation, shortest path first computation, and partial
route computation.
•
You can configure the switch to generate a log message when an IS-IS adjacency changes state (up
or down).
•
If a link in the network has a maximum transmission unit (MTU) size of less than 1500 bytes, you
can lower the LSP MTU so that routing will still occur.
•
The partition avoidance router configuration command prevents an area from becoming partitioned
when full connectivity is lost among a Level1-2 border router, adjacent Level 1 routers, and end
hosts.
Beginning in privileged EXEC mode, follow these steps to configure IS-IS parameters:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
clns routing
Enable ISO connectionless routing on the switch.
Step 3
router isis
Specify the IS-IS routing protocol and enter router configuration mode.
Step 4
default-information originate
[route-map map-name]
(Optional) Force a default route into the IS-IS routing domain.If you enter
route-map map-name, the routing process generates the default route if the
route map is satisfied.
Step 5
ignore-lsp-errors
(Optional) Configure the router to ignore LSPs with internal checksum
errors, instead of purging the LSPs. This command is enabled by default
(corrupted LSPs are dropped). To purge the corrupted LSPs, enter the no
ignore-lsp-errors router configuration command.
Step 6
area-password password
(Optional Configure the area authentication password, which is inserted in
Level 1 (station router level) LSPs.
Step 7
domain-password password
(Optional) Configure the routing domain authentication password, which is
inserted in Level 2 (area router level) LSPs.
Step 8
summary-address address mask
[level-1 | level-1-2 | level-2]
(Optional) Create a summary of addresses for a given level.
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Step 9
Command
Purpose
set-overload-bit [on-startup
{seconds | wait-for-bgp}]
(Optional) Set an overload bit (a hippity bit) to allow other routers to ignore
the router in their shortest path first (SPF) calculations if the router is
having problems.
•
(Optional) on-startup—sets the overload bit only on startup. If
on-startup is not specified, the overload bit is set immediately and
remains set until you enter the no set-overload-bit command. If
on-startup is specified, you must enter a number of seconds or
wait-for-bgp.
•
seconds—When the on-startup keyword is configured, causes the
overload bit to be set upon system startup and remain set for this
number of seconds. The range is from 5 to 86400 seconds.
•
wait-for-bgp—When the on-startup keyword is configured, causes
the overload bit to be set upon system startup and remain set until BGP
has converged. If BGP does not signal IS-IS that it is converged, IS-IS
will turn off the overload bit after 10 minutes.
Step 10
lsp-refresh-interval seconds
(Optional) Set an LSP refresh interval in seconds. The range is from 1 to
65535 seconds. The default is to send LSP refreshes every 900 seconds
(15 minutes).
Step 11
max-lsp-lifetime seconds
(Optional) Set the maximum time that LSP packets remain in the router
database without being refreshed. The range is from 1 to 65535 seconds.
The default is 1200 seconds (20 minutes). After the specified time interval,
the LSP packet is deleted.
Step 12
lsp-gen-interval [level-1 | level-2]
lsp-max-wait [lsp-initial-wait
lsp-second-wait]
(Optional) Set the IS-IS LSP generation throttling timers:
Step 13
spf-interval [level-1 | level-2]
spf-max-wait [spf-initial-wait
spf-second-wait]
•
lsp-max-wait—the maximum interval (in seconds) between two
consecutive occurrences of an LSP being generated. The range is 1 to
120, the default is 5.
•
lsp-initial-wait—the initial LSP generation delay (in milliseconds).
The range is 1 to 10000; the default is 50.
•
lsp-second-wait—the hold time between the first and second LSP
generation (in milliseconds). The range is 1 to 10000; the default is
5000.
(Optional) Sets IS-IS shortest path first (SPF) throttling timers.
•
spf-max-wait—the maximum interval between consecutive SFPs (in
seconds). The range is 1 to 120, the default is 10.
•
spf-initial-wait—the initial SFP calculation after a topology change (in
milliseconds). The range is 1 to 10000; the default is 5500.
•
spf-second-wait—the holdtime between the first and second SFP
calculation (in milliseconds). The range is 1 to 10000; the default is
5500.
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Step 14
Command
Purpose
prc-interval prc-max-wait
[prc-initial-wait prc-second-wait]
(Optional) Sets IS-IS partial route computation (PRC) throttling timers.
•
prc-max-wait—the maximum interval (in seconds) between two
consecutive PRC calculations. The range is 1 to 120; the default is 5.
•
prc-initial-wait—the initial PRC calculation delay (in milliseconds)
after a topology change. The range is 1 to 10,000; the default is 2000.
•
prc-second-wait—the hold time between the first and second PRC
calculation (in milliseconds). The range is 1 to 10,000; the default is
5000.
Step 15
log-adjacency-changes [all]
(Optional) Set the router to log IS-IS adjacency state changes. Enter all to
include all changes generated by events that are not related to the
Intermediate System-to-Intermediate System Hellos, including End
System-to-Intermediate System PDUs and link state packets (LSPs).
Step 16
lsp-mtu size
(Optional) Specify the maximum LSP packet size in bytes. The range is 128
to 4352; the default is 1497 bytes.
Note
If any link in the network has a reduced MTU size, you must change
the LSP MTU size on all routers in the network.
Step 17
partition avoidance
(Optional) Causes an IS-IS Level 1-2 border router to stop advertising the
Level 1 area prefix into the Level 2 backbone when full connectivity is lost
among the border router, all adjacent level 1 routers, and end hosts.
Step 18
end
Return to privileged EXEC mode.
Step 19
show clns
Verify your entries.
Step 20
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To disable default route generation, use the no default-information originate router configuration
command. Use the no area-password or no domain-password router configuration command to disable
passwords. To disable LSP MTU settings, use the no lsp mtu router configuration command. To return
to the default conditions for summary addressing, LSP refresh interval, LSP lifetime, LSP timers, SFP
timers, and PRC timers, use the no form of the commands. Use the no partition avoidance router
configuration command to disable the output format.
Configuring IS-IS Interface Parameters
You can optionally configure certain interface-specific IS-IS parameters, independently from other
attached routers. However, if you change some values from the defaults, such as multipliers and time
intervals, it makes sense to also change them on multiple routers and interfaces. Most of the interface
parameters can be configured for level 1, level 2, or both.
These are some interface level parameters you can configure:
•
The default metric on the interface, which is used as a value for the IS-IS metric and assigned when
there is no quality of service (QoS) routing performed.
•
The hello interval (length of time between hello packets sent on the interface) or the default hello
packet multiplier used on the interface to determine the hold time sent in IS-IS hello packets. The
hold time determines how long a neighbor waits for another hello packet before declaring the
neighbor down. This determines how quickly a failed link or neighbor is detected so that routes can
be recalculated. Change the hello-multiplier in circumstances where hello packets are lost
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frequently and IS-IS adjacencies are failing unnecessarily. You can raise the hello multiplier and
lower the hello interval correspondingly to make the hello protocol more reliable without increasing
the time required to detect a link failure.
•
Other time intervals:
– Complete sequence number PDU (CSNP) interval. CSNPs are sent by the designated router to
maintain database synchronization
– Retransmission interval. This is the time between retransmission of IS-IS LSPs for
point-to-point links.
– IS-IS LSP retransmission throttle interval. This is the maximum rate (number of milliseconds
between packets) at which IS-IS LSPs are re-sent on point-to-point links This interval is
different from the retransmission interval, which is the time between successive retransmissions
of the same LSP
•
Designated router election priority, which allows you to reduce the number of adjacencies required
on a multiaccess network, which in turn reduces the amount of routing protocol traffic and the size
of the topology database.
•
The interface circuit type, which is the type of adjacency desired for neighbors on the specified
interface
•
Password authentication for the interface
Beginning in privileged EXEC mode, follow these steps to configure IS-IS interface parameters:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Specify the interface to be configured and enter interface configuration
mode. If the interface is not already configured as a Layer 3 interface, enter
the no switchport command to put it into Layer 3 mode.
Step 3
isis metric default-metric [level-1 |
level-2]
(Optional) Configure the metric (or cost) for the specified interface. The
range is from 0 to 63. The default is 10. If no level is entered, the default is
to apply to both Level 1 and Level 2 routers.
Step 4
isis hello-interval {seconds |
minimal} [level-1 | level-2]
(Optional) Specify the length of time between hello packets sent by the
switch. By default, a value three times the hello interval seconds is
advertised as the holdtime in the hello packets sent. With smaller hello
intervals, topological changes are detected faster, but there is more routing
traffic.
•
minimal—causes the system to compute the hello interval based on the
hello multiplier so that the resulting hold time is 1 second.
•
seconds—the range is from 1 to 65535. The default is 10 seconds.
Step 5
isis hello-multiplier multiplier
[level-1 | level-2]
(Optional) Specify the number of IS-IS hello packets a neighbor must miss
before the router should declare the adjacency as down. The range is from
3 to 1000. The default is 3. Using a smaller hello-multiplier causes fast
convergence, but can result in more routing instability.
Step 6
isis csnp-interval seconds [level-1 |
level-2]
(Optional) Configure the IS-IS complete sequence number PDU (CSNP)
interval for the interface. The range is from 0 to 65535. The default is
10 seconds.
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Command
Purpose
Step 7
isis retransmit-interval seconds
(Optional) Configure the number of seconds between retransmission of
IS-IS LSPs for point-to-point links. The value you specify should be an
integer greater than the expected round-trip delay between any two routers
on the network. The range is from 0 to 65535. The default is 5 seconds.
Step 8
isis retransmit-throttle-interval
milliseconds
(Optional) Configure the IS-IS LSP retransmission throttle interval, which
is the maximum rate (number of milliseconds between packets) at which
IS-IS LSPs will be re-sent on point-to-point links. The range is from 0 to
65535. The default is determined by the isis lsp-interval command.
Step 9
isis priority value [level-1 | level-2]
(Optional) Configure the priority to use for designated router election. The
range is from 0 to 127. The default is 64.
Step 10
isis circuit-type {level-1 | level-1-2 |
level-2-only}
(Optional) Configure the type of adjacency desired for neighbors on the
specified interface (specify the interface circuit type).
•
level-1—a Level 1 adjacency is established if there is at least one area
address common to both this node and its neighbors.
•
level-1-2—a Level 1 and 2 adjacency is established if the neighbor is
also configured as both Level 1 and Level 2 and there is at least one
area in common. If there is no area in common, a Level 2 adjacency is
established. This is the default.
•
level 2—a Level 2 adjacency is established. If the neighbor router is a
Level 1 router, no adjacency is established.
Step 11
isis password password [level-1 |
level-2]
(Optional) Configure the authentication password for an interface. By
default, authentication is disabled. Specifying Level 1 or Level 2 enables
the password only for Level 1 or Level 2 routing, respectively. If you do not
specify a level, the default is Level 1 and Level 2.
Step 12
end
Return to privileged EXEC mode.
Step 13
show clns interface interface-id
Verify your entries.
Step 14
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To return to the default settings, use the no forms of the commands.
Monitoring and Maintaining ISO IGRP and IS-IS
You can remove all contents of a CLNS cache or remove information for a particular neighbor or route.
You can display specific CLNS or IS-IS statistics, such as the contents of routing tables, caches, and
databases. You can also display information about specific interfaces, filters, or neighbors.
Table 41-13 lists the privileged EXEC commands for clearing and displaying ISO CLNS and IS-IS
routing. For explanations of the display fields, see the Cisco IOS Apollo Domain, Banyan VINES,
DECnet, ISO CLNS and XNS Command Reference, Release 12.2, use the Cisco IOS command reference
master index, or search online.
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Table 41-13
ISO CLNS and IS-IS Clear and Show Commands
Command
Purpose
clear clns cache
Clear and reinitialize the CLNS routing cache.
clear clns es-neighbors
Remove end system (ES) neighbor information from the adjacency database.
clear clns is-neighbors
Remove intermediate system (IS) neighbor information from the adjacency
database.
clear clns neighbors
Remove CLNS neighbor information from the adjacency database.
clear clns route
Remove dynamically derived CLNS routing information.
show clns
Display information about the CLNS network.
show clns cache
Display the entries in the CLNS routing cache.
show clns es-neighbors
Display ES neighbor entries, including the associated areas.
show clns filter-expr
Display filter expressions.
show clns filter-set
Display filter sets.
show clns interface [interface-id]
Display the CLNS-specific or ES-IS information about each interface.
show clns neighbor
Display information about IS-IS neighbors.
show clns protocol
List the protocol-specific information for each IS-IS or ISO IGRP routing
process in this router.
show clns route
Display all the destinations to which this router knows how to route CLNS
packets.
show clns traffic
Display information about the CLNS packets this router has seen.
show ip route isis
Display the current state of the ISIS IP routing table.
show isis database
Display the IS-IS link-state database.
show isis routes
Display the IS-IS Level 1 routing table.
show isis spf-log
Display a history of the shortest path first (SPF) calculations for IS-IS.
show isis topology
Display a list of all connected routers in all areas.
show route-map
Display all route maps configured or only the one specified.
trace clns destination
Discover the paths taken to a specified destination by packets in the network.
which-route {nsap-address | clns-name}
Display the routing table in which the specified CLNS destination is found.
Configuring Multi-VRF CE
Virtual Private Networks (VPNs) provide a secure way for customers to share bandwidth over an ISP
backbone network. A VPN is a collection of sites sharing a common routing table. A customer site is
connected to the service-provider network by one or more interfaces, and the service provider associates
each interface with a VPN routing table, called a VPN routing/forwarding (VRF) table.
The switch supports multiple VPN routing/forwarding (multi-VRF) instances in customer edge (CE)
devices (multi-VRF CE) when the it is running the IP services or advanced IP services feature set.
Multi-VRF CE allows a service provider to support two or more VPNs with overlapping IP addresses.
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Note
The switch does not use Multiprotocol Label Switching (MPLS) to support VPNs. For information about
MPLS VRF, see the Cisco IOS Switching Services Configuration Guide, Release 12.2.
•
Understanding Multi-VRF CE, page 41-76
•
Default Multi-VRF CE Configuration, page 41-78
•
Multi-VRF CE Configuration Guidelines, page 41-78
•
Configuring VRFs, page 41-79
•
Configuring VRF-Aware Services, page 41-80
•
Configuring Multicast VRFs, page 41-84
•
Configuring a VPN Routing Session, page 41-84
•
Configuring BGP PE to CE Routing Sessions, page 41-85
•
Multi-VRF CE Configuration Example, page 41-86
•
Displaying Multi-VRF CE Status, page 41-89
Understanding Multi-VRF CE
Multi-VRF CE is a feature that allows a service provider to support two or more VPNs, where IP
addresses can be overlapped among the VPNs. Multi-VRF CE uses input interfaces to distinguish routes
for different VPNs and forms virtual packet-forwarding tables by associating one or more Layer 3
interfaces with each VRF. Interfaces in a VRF can be either physical, such as Ethernet ports, or logical,
such as VLAN SVIs, but an interface cannot belong to more than one VRF at any time.
Note
Multi-VRF CE interfaces must be Layer 3 interfaces.
Multi-VRF CE includes these devices:
•
Customer edge (CE) devices provide customers access to the service-provider network over a data
link to one or more provider edge routers. The CE device advertises the site’s local routes to the
router and learns the remote VPN routes from it. A Catalyst 3750-X or 3560-X switch can be a CE.
•
Provider edge (PE) routers exchange routing information with CE devices by using static routing or
a routing protocol such as BGP, RIPv2, OSPF, or EIGRP. The PE is only required to maintain VPN
routes for those VPNs to which it is directly attached, eliminating the need for the PE to maintain
all of the service-provider VPN routes. Each PE router maintains a VRF for each of its directly
connected sites. Multiple interfaces on a PE router can be associated with a single VRF if all of these
sites participate in the same VPN. Each VPN is mapped to a specified VRF. After learning local
VPN routes from CEs, a PE router exchanges VPN routing information with other PE routers by
using internal BGP (IBPG).
•
Provider routers or core routers are any routers in the service provider network that do not attach to
CE devices.
With multi-VRF CE, multiple customers can share one CE, and only one physical link is used between
the CE and the PE. The shared CE maintains separate VRF tables for each customer and switches or
routes packets for each customer based on its own routing table. Multi-VRF CE extends limited PE
functionality to a CE device, giving it the ability to maintain separate VRF tables to extend the privacy
and security of a VPN to the branch office.
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Figure 41-6 shows a configuration using Catalyst 3750-X or 3560-X switches as multiple virtual CEs.
This scenario is suited for customers who have low bandwidth requirements for their VPN service, for
example, small companies. In this case, multi-VRF CE support is required in the Catalyst 3750-X or
3560-X switches. Because multi-VRF CE is a Layer 3 feature, each interface in a VRF must be a Layer 3
interface.
Figure 41-6
Switches Acting as Multiple Virtual CEs
VPN 1
VPN 1
CE1
PE1
PE2
CE2
Service
provider
VPN 2
CE = Customer-edge device
PE = Provider-edge device
101385
VPN 2
When the CE switch receives a command to add a Layer 3 interface to a VRF, it sets up the appropriate
mapping between the VLAN ID and the policy label (PL) in multi-VRF-CE-related data structures and
adds the VLAN ID and PL to the VLAN database.
When multi-VRF CE is configured, the Layer 3 forwarding table is conceptually partitioned into two
sections:
•
The multi-VRF CE routing section contains the routes from different VPNs.
•
The global routing section contains routes to non-VPN networks, such as the Internet.
VLAN IDs from different VRFs are mapped into different policy labels, which are used to distinguish
the VRFs during processing. For each new VPN route learned, the Layer 3 setup function retrieves the
policy label by using the VLAN ID of the ingress port and inserts the policy label and new route to the
multi-VRF CE routing section. If the packet is received from a routed port, the port internal VLAN ID
number is used; if the packet is received from an SVI, the VLAN number is used.
This is the packet-forwarding process in a multi-VRF-CE-enabled network:
•
When the switch receives a packet from a VPN, the switch looks up the routing table based on the
input policy label number. When a route is found, the switch forwards the packet to the PE.
•
When the ingress PE receives a packet from the CE, it performs a VRF lookup. When a route is
found, the router adds a corresponding MPLS label to the packet and sends it to the MPLS network.
•
When an egress PE receives a packet from the network, it strips the label and uses the label to
identify the correct VPN routing table. Then it performs the normal route lookup. When a route is
found, it forwards the packet to the correct adjacency.
•
When a CE receives a packet from an egress PE, it uses the input policy label to look up the correct
VPN routing table. If a route is found, it forwards the packet within the VPN.
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To configure VRF, you create a VRF table and specify the Layer 3 interface associated with the VRF.
Then configure the routing protocols in the VPN and between the CE and the PE. BGP is the preferred
routing protocol used to distribute VPN routing information across the provider’s backbone. The
multi-VRF CE network has three major components:
•
VPN route target communities—lists of all other members of a VPN community. You need to
configure VPN route targets for each VPN community member.
•
Multiprotocol BGP peering of VPN community PE routers—propagates VRF reachability
information to all members of a VPN community. You need to configure BGP peering in all PE
routers within a VPN community.
•
VPN forwarding—transports all traffic between all VPN community members across a VPN
service-provider network.
Default Multi-VRF CE Configuration
Table 41-14
Default VRF Configuration
Feature
Default Setting
VRF
Disabled. No VRFs are defined.
Maps
No import maps, export maps, or route maps are defined.
VRF maximum routes
Fast Ethernet switches: 8000
Gigabit Ethernet switches: 12000.
Forwarding table
The default for an interface is the global routing table.
Multi-VRF CE Configuration Guidelines
Note
To use multi-VRF CE, you must have the IP services or advanced IP services feature set enabled on your
switch.
•
A switch with multi-VRF CE is shared by multiple customers, and each customer has its own routing
table.
•
Because customers use different VRF tables, the same IP addresses can be reused. Overlapped IP
addresses are allowed in different VPNs.
•
Multi-VRF CE lets multiple customers share the same physical link between the PE and the CE.
Trunk ports with multiple VLANs separate packets among customers. Each customer has its own
VLAN.
•
Multi-VRF CE does not support all MPLS-VRF functionality. It does not support label exchange,
LDP adjacency, or labeled packets.
•
For the PE router, there is no difference between using multi-VRF CE or using multiple CEs. In
Figure 41-6, multiple virtual Layer 3 interfaces are connected to the multi-VRF CE device.
•
The switch supports configuring VRF by using physical ports, VLAN SVIs, or a combination of
both. The SVIs can be connected through an access port or a trunk port.
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•
A customer can use multiple VLANs as long as they do not overlap with those of other customers.
A customer’s VLANs are mapped to a specific routing table ID that is used to identify the
appropriate routing tables stored on the switch.
•
The switch supports one global network and up to 26 VRFs.
•
Most routing protocols (BGP, OSPF, RIP, and static routing) can be used between the CE and the
PE. However, we recommend using external BGP (EBGP) for these reasons:
– BGP does not require multiple algorithms to communicate with multiple CEs.
– BGP is designed for passing routing information between systems run by different
administrations.
– BGP makes it easy to pass attributes of the routes to the CE.
•
Multi-VRF CE does not affect the packet switching rate.
•
VPN multicast is not supported.
•
You can configure 104 policies whether or not VRFs are configured on the switch or switch stack.
•
You can enable VRF on a private VLAN, and the reverse.
•
You cannot enable VRF when policy-based routing (PBR) is enabled on an interface, and the
reverse.
•
You cannot enable VRF when Web Cache Communication Protocol (WCCP) is enabled on an
interface, and the reverse.
Configuring VRFs
Beginning in privileged EXEC mode, follow these steps to configure one or more VRFs. For complete
syntax and usage information for the commands, see the switch command reference for this release and
the Cisco IOS Switching Services Command Reference, Release 12.2.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip routing
Enable IP routing.
Step 3
ip vrf vrf-name
Name the VRF, and enter VRF configuration mode.
Step 4
rd route-distinguisher
Create a VRF table by specifying a route distinguisher. Enter either an
AS number and an arbitrary number (xxx:y) or an IP address and
arbitrary number (A.B.C.D:y)
Step 5
route-target {export | import | both}
route-target-ext-community
Create a list of import, export, or import and export route target
communities for the specified VRF. Enter either an AS system number
and an arbitrary number (xxx:y) or an IP address and an arbitrary
number (A.B.C.D:y). The route-target-ext-community should be the
same as the route-distinguisher entered in Step 4.
Step 6
import map route-map
(Optional) Associate a route map with the VRF.
Step 7
interface interface-id
Specify the Layer 3 interface to be associated with the VRF, and enter
interface configuration mode. The interface can be a routed port or SVI.
Step 8
ip vrf forwarding vrf-name
Associate the VRF with the Layer 3 interface.
Step 9
end
Return to privileged EXEC mode.
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Command
Purpose
Step 10
show ip vrf [brief | detail | interfaces]
[vrf-name]
Verify the configuration. Display information about the configured
VRFs.
Step 11
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no ip vrf vrf-name global configuration command to delete a VRF and to remove all interfaces
from it. Use the no ip vrf forwarding interface configuration command to remove an interface from the
VRF.
Configuring VRF-Aware Services
IP services can be configured on global interfaces, and these services run within the global routing
instance. IP services are enhanced to run on multiple routing instances; they are VRF-aware. Any
configured VRF in the system can be specified for a VRF-aware service.
VRF-Aware services are implemented in platform-independent modules. VRF means multiple routing
instances in Cisco IOS. Each platform has its own limit on the number of VRFs it supports.
VRF-aware services have the following characteristics:
•
The user can ping a host in a user-specified VRF.
•
ARP entries are learned in separate VRFs. The user can display Address Resolution Protocol (ARP)
entries for specific VRFs.
These services are VRF-Aware:
•
ARP
•
Ping
•
Simple Network Management Protocol (SNMP)
•
Hot Standby Router Protocol (HSRP)
•
Unicast Reverse Path Forwarding (uRPF)
•
Syslog
•
Traceroute
•
FTP and TFTP
User Interface for ARP
Beginning in privileged EXEC mode, follow these steps to configure VRF-aware services for ARP. For
complete syntax and usage information for the commands, refer to the switch command reference for
this release and the Cisco IOS Switching Services Command Reference, Release 12.2.
Command
Purpose
show ip arp vrf vrf-name
Display the ARP table in the specified VRF.
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User Interface for PING
Beginning in privileged EXEC mode, follow these steps to configure VRF-aware services for ping. For
complete syntax and usage information for the commands, refer to the switch command reference for
this release and the Cisco IOS Switching Services Command Reference, Release 12.2.
Command
Purpose
ping vrf vrf-name ip-host
Display the ARP table in the specified VRF.
User Interface for SNMP
Beginning in privileged EXEC mode, follow these steps to configure VRF-aware services for SNMP. For
complete syntax and usage information for the commands, refer to the switch command reference for
this release and the Cisco IOS Switching Services Command Reference, Release 12.2.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
snmp-server trap authentication vrf
Enable SNMP traps for packets on a VRF.
Step 3
snmp-server engineID remote host vrf
vpn-instance engine-id string
Configure a name for the remote SNMP engine on a switch.
Step 4
snmp-server host host vrf vpn-instance
traps community
Specify the recipient of an SNMP trap operation and specify the VRF
table to be used for sending SNMP traps.
Step 5
snmp-server host host vrf vpn-instance
informs community
Specify the recipient of an SNMP inform operation and specify the VRF
table to be used for sending SNMP informs.
Step 6
snmp-server user user group remote host Add a user to an SNMP group for a remote host on a VRF for SNMP
vrf vpn- instance security model
access.
Step 7
end
Return to privileged EXEC mode.
User Interface for HSRP
HSRP support for VRFs ensures that HSRP virtual IP addresses are added to the correct IP routing table.
Beginning in privileged EXEC mode, follow these steps to configure VRF-aware services for HSRP. For
complete syntax and usage information for the commands, refer to the switch command reference for
this release and the Cisco IOS Switching Services Command Reference, Release 12.2.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the Layer 3 interface to
configure.
Step 3
no switchport
Remove the interface from Layer 2 configuration mode if it is a physical
interface.
Step 4
ip vrf forwarding vrf-name
Configure VRF on the interface.
Step 5
ip address ip- address
Enter the IP address for the interface.
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Command
Purpose
Step 6
standby 1 ip ip-address
Enable HSRP and configure the virtual IP address.
Step 7
end
Return to privileged EXEC mode.
User Interface for uRPF
uRPF can be configured on an interface assigned to a VRF, and source lookup is done in the VRF table.
Beginning in privileged EXEC mode, follow these steps to configure VRF-aware services for uRPF. For
complete syntax and usage information for the commands, refer to the switch command reference for
this release and the Cisco IOS Switching Services Command Reference, Release 12.2.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
interface interface-id
Enter interface configuration mode, and specify the Layer 3 interface to
configure.
Step 3
no switchport
Remove the interface from Layer 2 configuration mode if it is a physical
interface.
Step 4
ip vrf forwarding vrf-name
Configure VRF on the interface.
Step 5
ip address ip-address
Enter the IP address for the interface.
Step 6
ip verify unicast reverse-path
Enable uRPF on the interface.
Step 7
end
Return to privileged EXEC mode.
User Interface for VRF-Aware RADIUS
To configure VRF-Aware RADIUS, you must first enable AAA on a RADIUS server. The switch
supports the ip vrf forwarding vrf-name server-group configuration and the ip radius source-interface
global configuration commands, as described in the Per VRF AAA Feature Guide at this URL:
http://www.cisco.com/en/US/docs/ios/12_2t/12_2t13/feature/guide/ftvrfaaa.html
User Interface for Syslog
Beginning in privileged EXEC mode, follow these steps to configure VRF-aware services for Syslog.
For complete syntax and usage information for the commands, refer to the switch command reference
for this release and the Cisco IOS Switching Services Command Reference, Release 12.2.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
logging on
Enable or temporarily disable logging of storage router event message.
Step 3
logging host ip-address vrf vrf-name
Specify the host address of the syslog server where logging messages
are to be sent.
Step 4
logging buffered logging buffered size
debugging
Log messages to an internal buffer.
Step 5
logging trap debugging
Limit the logging messages sent to the syslog server.
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Command
Purpose
Step 6
logging facility facility
Send system logging messages to a logging facility.
Step 7
end
Return to privileged EXEC mode.
User Interface for Traceroute
Beginning in privileged EXEC mode, follow these steps to configure VRF-aware services for traceroute.
For complete syntax and usage information for the commands, refer to the switch command reference
for this release and the Cisco IOS Switching Services Command Reference, Release 12.2.
Command
Purpose
traceroute vrf vrf-name ipaddress
Specify the name of a VPN VRF in which to find the destination
address.
User Interface for FTP and TFTP
So that FTP and TFTP are VRF-aware, you must configure some FTP/TFTP CLIs. For example, if you
want to use a VRF table that is attached to an interface, say E1/0, you need to configure the CLI ip [t]ftp
source-interface E1/0 to inform [t]ftp to use a specific routing table. In this example, the VRF table is
used to look up the destination IP address. These changes are backward-compatible and do not affect
existing behavior. That is, you can use the source-interface CLI to send packets out a particular interface
even if no VRF is configured on that interface.
To specify the source IP address for FTP connections, use the ip ftp source-interface show mode
command. To use the address of the interface where the connection is made, use the no form of this
command.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip ftp source-interface interface-type
interface-number
Specify the source IP address for FTP connections.
Step 3
end
Return to privileged EXEC mode.
To specify the IP address of an interface as the source address for TFTP connections, use the ip tftp
source-interface show mode command. To return to the default, use the no form of this command.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip tftp source-interface interface-type
interface-number
Specify the source IP address for TFTP connections.
Step 3
end
Return to privileged EXEC mode.
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Configuring Multicast VRFs
Beginning in privileged EXEC mode, follow these steps to configure a multicast within a VRF table. For
complete syntax and usage information for the commands, see the switch command reference for this
release and the Cisco IOS Switching Services Command Reference, Release 12.2.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip routing
Enable IP routing mode.
Step 3
ip vrf vrf-name
Name the VRF, and enter VRF configuration mode.
Step 4
rd route-distinguisher
Create a VRF table by specifying a route distinguisher. Enter either an
AS number and an arbitrary number (xxx:y) or an IP address and an
arbitrary number (A.B.C.D:y)
Step 5
route-target {export | import | both}
route-target-ext-community
Create a list of import, export, or import and export route target
communities for the specified VRF. Enter either an AS system number
and an arbitrary number (xxx:y) or an IP address and an arbitrary
number (A.B.C.D:y). The route-target-ext-community should be the
same as the route-distinguisher entered in Step 4.
Step 6
import map route-map
(Optional) Associate a route map with the VRF.
Step 7
ip multicast-routing vrf vrf-name
distributed
(Optional) Enable global multicast routing for VRF table.
Step 8
interface interface-id
Specify the Layer 3 interface to be associated with the VRF, and enter
interface configuration mode. The interface can be a routed port or an
SVI.
Step 9
ip vrf forwarding vrf-name
Associate the VRF with the Layer 3 interface.
Step 10
ip address ip-address mask
Configure IP address for the Layer 3 interface.
Step 11
ip pim sparse-dense mode
Enable PIM on the VRF-associated Layer 3 interface.
Step 12
end
Return to privileged EXEC mode.
Step 13
show ip vrf [brief | detail | interfaces]
[vrf-name]
Verify the configuration. Display information about the configured
VRFs.
Step 14
copy running-config startup-config
(Optional) Save your entries in the configuration file.
For more information about configuring a multicast within a Multi-VRF CE, see the Cisco IOS IP
Multicast Configuration Guide, Release 12.4.
Configuring a VPN Routing Session
Routing within the VPN can be configured with any supported routing protocol (RIP, OSPF, EIGRP, or
BGP) or with static routing. The configuration shown here is for OSPF, but the process is the same for
other protocols.
Note
To configure an EIGRP routing process to run within a VRF instance, you must configure an
autonomous-system number by entering the autonomous-system autonomous-system-number
address-family configuration mode command.
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Beginning in privileged EXEC mode, follow these steps to configure OSPF in the VPN:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router ospf process-id vrf vrf-name
Enable OSPF routing, specify a VPN forwarding table, and enter router
configuration mode.
Step 3
log-adjacency-changes
(Optional) Log changes in the adjacency state. This is the default state.
Step 4
redistribute bgp
autonomous-system-number subnets
Set the switch to redistribute information from the BGP network to the
OSPF network.
Step 5
network network-number area area-id
Define a network address and mask on which OSPF runs and the area ID
for that network address.
Step 6
end
Return to privileged EXEC mode.
Step 7
show ip ospf process-id
Verify the configuration of the OSPF network.
Step 8
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no router ospf process-id vrf vrf-name global configuration command to disassociate the VPN
forwarding table from the OSPF routing process.
Configuring BGP PE to CE Routing Sessions
Beginning in privileged EXEC mode, follow these steps to configure a BGP PE to CE routing session:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router bgp autonomous-system-number
Configure the BGP routing process with the AS number passed to other
BGP routers, and enter router configuration mode.
Step 3
network network-number mask
network-mask
Specify a network and mask to announce using BGP.
Step 4
redistribute ospf process-id match
internal
Set the switch to redistribute OSPF internal routes.
Step 5
network network-number area area-id
Define a network address and mask on which OSPF runs and the area ID
for that network address.
Step 6
address-family ipv4 vrf vrf-name
Define BGP parameters for PE to CE routing sessions, and enter VRF
address-family mode.
Step 7
neighbor address remote-as as-number
Define a BGP session between PE and CE routers.
Step 8
neighbor address activate
Activate the advertisement of the IPv4 address family.
Step 9
end
Return to privileged EXEC mode.
Step 10
show ip bgp [ipv4] [neighbors]
Verify BGP configuration.
Step 11
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no router bgp autonomous-system-number global configuration command to delete the BGP
routing process. Use the command with keywords to delete routing characteristics.
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Multi-VRF CE Configuration Example
Figure 41-7 is a simplified example of the physical connections in a network similar to that in
Figure 41-6. OSPF is the protocol used in VPN1, VPN2, and the global network. BGP is used in the CE
to PE connections. The examples following the illustration show how to configure a switch as CE
Switch A, and the VRF configuration for customer switches D and F. Commands for configuring CE
Switch C and the other customer switches are not included but would be similar. The example also
includes commands for configuring traffic to Switch A for a Catalyst 6000 or Catalyst 6500 switch acting
as a PE router.
Figure 41-7
Multi-VRF CE Configuration Example
Switch A
Switch B
Switch C
VPN1
Switch D
VPN1
208.0.0.0
Fast
Ethernet
8
Switch H
Switch E
108.0.0.0
VPN2
Fast
Ethernet
7
CE1
Switch F
118.0.0.0
Fast
Ethernet
11
VPN2
PE
CE2
Switch J
Gigabit
Ethernet
1
Global network
Switch K
Global network
168.0.0.0
Fast
Ethernet
3
CE = Customer-edge device
PE = Provider-edge device
101386
Switch G
Configuring Switch A
On Switch A, enable routing and configure VRF.
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# ip routing
Switch(config)# ip vrf v11
Switch(config-vrf)# rd 800:1
Switch(config-vrf)# route-target export 800:1
Switch(config-vrf)# route-target import 800:1
Switch(config-vrf)# exit
Switch(config)# ip vrf v12
Switch(config-vrf)# rd 800:2
Switch(config-vrf)# route-target export 800:2
Switch(config-vrf)# route-target import 800:2
Switch(config-vrf)# exit
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Configure the loopback and physical interfaces on Switch A. Gigabit Ethernet port 1 is a trunk
connection to the PE. Gigabit Ethernet ports 8 and 11 connect to VPNs:
Switch(config)# interface loopback1
Switch(config-if)# ip vrf forwarding v11
Switch(config-if)# ip address 8.8.1.8 255.255.255.0
Switch(config-if)# exit
Switch(config)# interface loopback2
Switch(config-if)# ip vrf forwarding v12
Switch(config-if)# ip address 8.8.2.8 255.255.255.0
Switch(config-if)# exit
Switch(config)# interface gigabitethernet1/0/5
Switch(config-if)# switchport trunk encapsulation dot1q
Switch(config-if)# switchport mode trunk
Switch(config-if)# no ip address
Switch(config-if)# exit
Switch(config)# interface gigabitethernet1/0/8
Switch(config-if)# switchport access vlan 208
Switch(config-if)# no ip address
Switch(config-if)# exit
Switch(config)# interface gigabitethernet1/0/11
Switch(config-if)# switchport trunk encapsulation dot1q
Switch(config-if)# switchport mode trunk
Switch(config-if)# no ip address
Switch(config-if)# exit
Configure the VLANs used on Switch A. VLAN 10 is used by VRF 11 between the CE and the PE.
VLAN 20 is used by VRF 12 between the CE and the PE. VLANs 118 and 208 are used for the VPNs
that include Switch F and Switch D, respectively:
Switch(config)# interface vlan10
Switch(config-if)# ip vrf forwarding v11
Switch(config-if)# ip address 38.0.0.8 255.255.255.0
Switch(config-if)# exit
Switch(config)# interface vlan20
Switch(config-if)# ip vrf forwarding v12
Switch(config-if)# ip address 83.0.0.8 255.255.255.0
Switch(config-if)# exit
Switch(config)# interface vlan118
Switch(config-if)# ip vrf forwarding v12
Switch(config-if)# ip address 118.0.0.8 255.255.255.0
Switch(config-if)# exit
Switch(config)# interface vlan208
Switch(config-if)# ip vrf forwarding v11
Switch(config-if)# ip address 208.0.0.8 255.255.255.0
Switch(config-if)# exit
Configure OSPF routing in VPN1 and VPN2.
Switch(config)# router
Switch(config-router)#
Switch(config-router)#
Switch(config-router)#
Switch(config)# router
Switch(config-router)#
Switch(config-router)#
Switch(config-router)#
ospf 1 vrf vl1
redistribute bgp 800 subnets
network 208.0.0.0 0.0.0.255 area 0
exit
ospf 2 vrf vl2
redistribute bgp 800 subnets
network 118.0.0.0 0.0.0.255 area 0
exit
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Configure BGP for CE to PE routing.
Switch(config)# router bgp 800
Switch(config-router)# address-family ipv4 vrf vl2
Switch(config-router-af)# redistribute ospf 2 match internal
Switch(config-router-af)# neighbor 83.0.0.3 remote-as 100
Switch(config-router-af)# neighbor 83.0.0.3 activate
Switch(config-router-af)# network 8.8.2.0 mask 255.255.255.0
Switch(config-router-af)# exit
Switch(config-router)# address-family ipv4 vrf vl1
Switch(config-router-af)# redistribute ospf 1 match internal
Switch(config-router-af)# neighbor 38.0.0.3 remote-as 100
Switch(config-router-af)# neighbor 38.0.0.3 activate
Switch(config-router-af)# network 8.8.1.0 mask 255.255.255.0
Switch(config-router-af)# end
Configuring Switch D
Switch D belongs to VPN 1. Configure the connection to Switch A by using these commands.
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# ip routing
Switch(config)# interface gigabitethernet1/0/2
Switch(config-if)# no switchport
Switch(config-if)# ip address 208.0.0.20 255.255.255.0
Switch(config-if)# exit
Switch(config)# router ospf 101
Switch(config-router)# network 208.0.0.0 0.0.0.255 area 0
Switch(config-router)# end
Configuring Switch F
Switch F belongs to VPN 2. Configure the connection to Switch A by using these commands.
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# ip routing
Switch(config)# interface gigabitethernet1/0/1
Switch(config-if)# switchport trunk encapsulation dot1q
Switch(config-if)# switchport mode trunk
Switch(config-if)# no ip address
Switch(config-if)# exit
Switch(config)# interface vlan118
Switch(config-if)# ip address 118.0.0.11 255.255.255.0
Switch(config-if)# exit
Switch(config)# router ospf 101
Switch(config-router)# network 118.0.0.0 0.0.0.255 area 0
Switch(config-router)# end
Configuring the PE Switch B
When used on switch B (the PE router), these commands configure only the connections to the CE
device, Switch A.
Router# configure terminal
Enter configuration commands, one per line.
Router(config)# ip vrf v1
End with CNTL/Z.
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Router(config-vrf)#
Router(config-vrf)#
Router(config-vrf)#
Router(config-vrf)#
rd 100:1
route-target export 100:1
route-target import 100:1
exit
Router(config)# ip vrf v2
Router(config-vrf)# rd 100:2
Router(config-vrf)# route-target export 100:2
Router(config-vrf)# route-target import 100:2
Router(config-vrf)# exit
Router(config)# ip cef
Router(config)# interface Loopback1
Router(config-if)# ip vrf forwarding v1
Router(config-if)# ip address 3.3.1.3 255.255.255.0
Router(config-if)# exit
Router(config)# interface Loopback2
Router(config-if)# ip vrf forwarding v2
Router(config-if)# ip address 3.3.2.3 255.255.255.0
Router(config-if)# exit
Router(config)# interface gigabitethernet1/1/0.10
Router(config-if)# encapsulation dot1q 10
Router(config-if)# ip vrf forwarding v1
Router(config-if)# ip address 38.0.0.3 255.255.255.0
Router(config-if)# exit
Router(config)# interface gigabitethernet1/1/0.20
Router(config-if)# encapsulation dot1q 20
Router(config-if)# ip vrf forwarding v2
Router(config-if)# ip address 83.0.0.3 255.255.255.0
Router(config-if)# exit
Router(config)# router bgp 100
Router(config-router)# address-family ipv4 vrf v2
Router(config-router-af)# neighbor 83.0.0.8 remote-as 800
Router(config-router-af)# neighbor 83.0.0.8 activate
Router(config-router-af)# network 3.3.2.0 mask 255.255.255.0
Router(config-router-af)# exit
Router(config-router)# address-family ipv4 vrf vl
Router(config-router-af)# neighbor 38.0.0.8 remote-as 800
Router(config-router-af)# neighbor 38.0.0.8 activate
Router(config-router-af)# network 3.3.1.0 mask 255.255.255.0
Router(config-router-af)# end
Displaying Multi-VRF CE Status
Table 41-15
Commands for Displaying Multi-VRF CE Information
Command
Purpose
show ip protocols vrf vrf-name
Display routing protocol information associated
with a VRF.
show ip route vrf vrf-name [connected] [protocol [as-number]] [list]
[mobile] [odr] [profile] [static] [summary] [supernets-only]
Display IP routing table information associated
with a VRF.
show ip vrf [brief | detail | interfaces] [vrf-name]
Display information about the defined VRF
instances.
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Configuring Unicast Reverse Path Forwarding
For more information about the information in the displays, see the Cisco IOS Switching Services
Command Reference, Release 12.2.
Configuring Unicast Reverse Path Forwarding
The unicast reverse path forwarding (unicast RPF) feature helps to mitigate problems that are caused by
the introduction of malformed or forged (spoofed) IP source addresses into a network by discarding IP
packets that lack a verifiable IP source address. For example, a number of common types of
denial-of-service (DoS) attacks, including Smurf and Tribal Flood Network (TFN), can take advantage
of forged or rapidly changing source IP addresses to allow attackers to thwart efforts to locate or filter
the attacks. For Internet service providers (ISPs) that provide public access, Unicast RPF deflects such
attacks by forwarding only packets that have source addresses that are valid and consistent with the IP
routing table. This action protects the network of the ISP, its customer, and the rest of the Internet.
Note
Do not configure unicast RPF if the switch is in a mixed hardware stack combining more than one switch
type: Catalyst 3750-X, Catalyst 3750-E, and Catalyst 3750 switches.
For detailed IP unicast RPF configuration information, see the Other Security Features chapter in the
Cisco IOS Security Configuration Guide, Release 12.2.
Configuring Protocol-Independent Features
This section describes how to configure IP routing protocol-independent features. These features are
available on switches running the IP base or the IP services feature set; except that with the IP base
feature set, protocol-related features are available only for RIP. For a complete description of the IP
routing protocol-independent commands in this chapter, see the “IP Routing Protocol-Independent
Commands” chapter of the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols,
Release 12.2.
•
Configuring Distributed Cisco Express Forwarding, page 41-90
•
Configuring the Number of Equal-Cost Routing Paths, page 41-92
•
Configuring Static Unicast Routes, page 41-93
•
Specifying Default Routes and Networks, page 41-94
•
Using Route Maps to Redistribute Routing Information, page 41-94
•
Configuring Policy-Based Routing, page 41-98
•
Filtering Routing Information, page 41-101
•
Managing Authentication Keys, page 41-104
Configuring Distributed Cisco Express Forwarding
Cisco Express Forwarding (CEF) is a Layer 3 IP switching technology used to optimize network
performance. CEF implements an advanced IP look-up and forwarding algorithm to deliver maximum
Layer 3 switching performance. CEF is less CPU-intensive than fast switching route caching, allowing
more CPU processing power to be dedicated to packet forwarding. In a switch stack, the hardware uses
distributed CEF (dCEF) in the stack. In dynamic networks, fast switching cache entries are frequently
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invalidated because of routing changes, which can cause traffic to be process switched using the routing
table, instead of fast switched using the route cache. CEF and dCEF use the Forwarding Information
Base (FIB) lookup table to perform destination-based switching of IP packets.
The two main components in CEF and dCEF are the distributed FIB and the distributed adjacency tables.
•
The FIB is similar to a routing table or information base and maintains a mirror image of the
forwarding information in the IP routing table. When routing or topology changes occur in the
network, the IP routing table is updated, and those changes are reflected in the FIB. The FIB
maintains next-hop address information based on the information in the IP routing table. Because
the FIB contains all known routes that exist in the routing table, CEF eliminates route cache
maintenance, is more efficient for switching traffic, and is not affected by traffic patterns.
•
Nodes in the network are said to be adjacent if they can reach each other with a single hop across a
link layer. CEF uses adjacency tables to prepend Layer 2 addressing information. The adjacency
table maintains Layer 2 next-hop addresses for all FIB entries.
Because the switch or switch stack uses Application Specific Integrated Circuits (ASICs) to achieve
Gigabit-speed line rate IP traffic, CEF or dCEF forwarding applies only to the software-forwarding path,
that is, traffic that is forwarded by the CPU.
CEF or distributed CEF is enabled globally by default. If for some reason it is disabled, you can re-enable
it by using the ip cef or ip cef distributed global configuration command.
The default configuration is CEF or dCEF enabled on all Layer 3 interfaces. Entering the no ip
route-cache cef interface configuration command disables CEF for traffic that is being forwarded by
software. This command does not affect the hardware forwarding path. Disabling CEF and using the
debug ip packet detail privileged EXEC command can be useful to debug software-forwarded traffic.
To enable CEF on an interface for the software-forwarding path, use the ip route-cache cef interface
configuration command.
Caution
Although the no ip route-cache cef interface configuration command to disable CEF on an interface is
visible in the CLI, we strongly recommend that you do not disable CEF or dCEF on interfaces except
for debugging purposes.
Beginning in privileged EXEC mode, follow these steps to enable CEF or dCEF globally and on an
interface for software-forwarded traffic if it has been disabled:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip cef
Enable CEF operation on a Catalyst 3560-X switch,
or
or
ip cef distributed
enable CEF operation on a Catalyst 3750-X switch.
Step 3
interface interface-id
Enter interface configuration mode, and specify the Layer 3
interface to configure.
Step 4
ip route-cache cef
Enable CEF on the interface for software-forwarded traffic.
Step 5
end
Return to privileged EXEC mode.
Step 6
show ip cef
Display the CEF status on all interfaces.
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Step 7
Command
Purpose
show cef linecard [detail]
Display CEF-related interface information on a Catalyst
3560-X switch, or
or
show cef linecard [slot-number] [detail]
Display CEF-related interface information on a Catalyst
3750-X switch by stack member for all switches in the stack or
for the specified switch.
(Optional) For slot-number, enter the stack member switch
number.
Step 8
show cef interface [interface-id]
Display detailed CEF information for all interfaces or the
specified interface.
Step 9
show adjacency
Display CEF adjacency table information.
Step 10
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Configuring the Number of Equal-Cost Routing Paths
When a router has two or more routes to the same network with the same metrics, these routes can be
thought of as having an equal cost. The term parallel path is another way to see occurrences of equal-cost
routes in a routing table. If a router has two or more equal-cost paths to a network, it can use them
concurrently. Parallel paths provide redundancy in case of a circuit failure and also enable a router to
load balance packets over the available paths for more efficient use of available bandwidth. Equal-cost
routes are supported across switches in a stack.
Even though the router automatically learns about and configures equal-cost routes, you can control the
maximum number of parallel paths supported by an IP routing protocol in its routing table. Although the
switch software allows a maximum of 32 equal-cost routes, the switch hardware will never use more than
16 paths per route.
Beginning in privileged EXEC mode, follow these steps to change the maximum number of parallel
paths installed in a routing table from the default:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router {bgp | rip | ospf | eigrp}
Enter router configuration mode.
Step 3
maximum-paths maximum
Set the maximum number of parallel paths for the protocol routing
table. The range is from 1 to 16; the default is 4 for most IP routing
protocols, but only 1 for BGP.
Step 4
end
Return to privileged EXEC mode.
Step 5
show ip protocols
Verify the setting in the Maximum path field.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no maximum-paths router configuration command to restore the default value.
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Configuring Static Unicast Routes
Static unicast routes are user-defined routes that cause packets moving between a source and a
destination to take a specified path. Static routes can be important if the router cannot build a route to a
particular destination and are useful for specifying a gateway of last resort to which all unroutable
packets are sent.
Beginning in privileged EXEC mode, follow these steps to configure a static route:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip route prefix mask {address | interface} [distance]
Establish a static route.
Step 3
end
Return to privileged EXEC mode.
Step 4
show ip route
Display the current state of the routing table to verify
the configuration.
Step 5
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no ip route prefix mask {address | interface} global configuration command to remove a static
route.
The switch retains static routes until you remove them. However, you can override static routes with
dynamic routing information by assigning administrative distance values. Each dynamic routing
protocol has a default administrative distance, as listed in Table 41-16. If you want a static route to be
overridden by information from a dynamic routing protocol, set the administrative distance of the static
route higher than that of the dynamic protocol.
Table 41-16
Dynamic Routing Protocol Default Administrative Distances
Route Source
Default Distance
Connected interface
0
Static route
1
Enhanced IRGP summary route
5
External BGP
20
Internal Enhanced IGRP
90
IGRP
100
OSPF
110
Internal BGP
200
Unknown
225
Static routes that point to an interface are advertised through RIP, IGRP, and other dynamic routing
protocols, whether or not static redistribute router configuration commands were specified for those
routing protocols. These static routes are advertised because static routes that point to an interface are
considered in the routing table to be connected and hence lose their static nature. However, if you define
a static route to an interface that is not one of the networks defined in a network command, no dynamic
routing protocols advertise the route unless a redistribute static command is specified for these
protocols.
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When an interface goes down, all static routes through that interface are removed from the IP routing
table. When the software can no longer find a valid next hop for the address specified as the forwarding
router's address in a static route, the static route is also removed from the IP routing table.
Specifying Default Routes and Networks
A router might not be able to learn the routes to all other networks. To provide complete routing
capability, you can use some routers as smart routers and give the remaining routers default routes to the
smart router. (Smart routers have routing table information for the entire internetwork.) These default
routes can be dynamically learned or can be configured in the individual routers. Most dynamic interior
routing protocols include a mechanism for causing a smart router to generate dynamic default
information that is then forwarded to other routers.
If a router has a directly connected interface to the specified default network, the dynamic routing
protocols running on that device generate a default route. In RIP, it advertises the pseudonetwork 0.0.0.0.
A router that is generating the default for a network also might need a default of its own. One way a
router can generate its own default is to specify a static route to the network 0.0.0.0 through the
appropriate device.
Beginning in privileged EXEC mode, follow these steps to define a static route to a network as the static
default route:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip default-network network number
Specify a default network.
Step 3
end
Return to privileged EXEC mode.
Step 4
show ip route
Display the selected default route in the gateway of last resort
display.
Step 5
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no ip default-network network number global configuration command to remove the route.
When default information is passed through a dynamic routing protocol, no further configuration is
required. The system periodically scans its routing table to choose the optimal default network as its
default route. In IGRP networks, there might be several candidate networks for the system default. Cisco
routers use administrative distance and metric information to set the default route or the gateway of last
resort.
If dynamic default information is not being passed to the system, candidates for the default route are
specified with the ip default-network global configuration command. If this network appears in the
routing table from any source, it is flagged as a possible choice for the default route. If the router has no
interface on the default network, but does have a path to it, the network is considered as a possible
candidate, and the gateway to the best default path becomes the gateway of last resort.
Using Route Maps to Redistribute Routing Information
The switch can run multiple routing protocols simultaneously, and it can redistribute information from
one routing protocol to another. Redistributing information from one routing protocol to another applies
to all supported IP-based routing protocols.
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You can also conditionally control the redistribution of routes between routing domains by defining
enhanced packet filters or route maps between the two domains. The match and set route-map
configuration commands define the condition portion of a route map. The match command specifies that
a criterion must be matched. The set command specifies an action to be taken if the routing update meets
the conditions defined by the match command. Although redistribution is a protocol-independent
feature, some of the match and set route-map configuration commands are specific to a particular
protocol.
One or more match commands and one or more set commands follow a route-map command. If there
are no match commands, everything matches. If there are no set commands, nothing is done, other than
the match. Therefore, you need at least one match or set command.
Note
A route map with no set route-map configuration commands is sent to the CPU, which could cause high
CPU utilization.
You can also identify route-map statements as permit or deny. If the statement is marked as a deny, the
packets meeting the match criteria are sent back through the normal forwarding channels
(destination-based routing). If the statement is marked as permit, set clauses are applied to packets
meeting the match criteria. Packets that do not meet the match criteria are forwarded through the normal
routing channel.
You can use the BGP route map continue clause to execute additional entries in a route map after an
entry is executed with successful match and set clauses. You can use the continue clause to configure
and organize more modular policy definitions so that specific policy configurations need not be repeated
within the same route map. Beginning in Cisco IOS Release 12.2(37)SE, the switch supports the
continue clause for outbound policies. For more information about using the route map continue clause,
see the BGP Route-Map Continue Support for an Outbound Policy feature guide for Cisco IOS
Release 12.4(4)T.
Although each of Steps 3 through 14 in the following section is optional, you must enter at least one
match route-map configuration command and one set route-map configuration command.
Beginning in privileged EXEC mode, follow these steps to configure a route map for redistribution:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
route-map map-tag [permit | deny]
[sequence number]
Define any route maps used to control redistribution and enter route-map
configuration mode.
map-tag—A meaningful name for the route map. The redistribute
router configuration command uses this name to reference this route
map. Multiple route maps might share the same map tag name.
(Optional) If permit is specified and the match criteria are met for this
route map, the route is redistributed as controlled by the set actions. If
deny is specified, the route is not redistributed.
sequence number (Optional)— Number that indicates the position a new
route map is to have in the list of route maps already configured with the
same name.
Step 3
match as-path path-list-number
Match a BGP AS path access list.
Step 4
match community-list
community-list-number [exact]
Match a BGP community list.
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Command
Purpose
Step 5
match ip address {access-list-number | Match a standard access list by specifying the name or number. It can be
access-list-name} [...access-list-number | an integer from 1 to 199.
...access-list-name]
Step 6
match metric metric-value
Step 7
match ip next-hop {access-list-number | Match a next-hop router address passed by one of the access lists
access-list-name} [...access-list-number | specified (numbered from 1 to 199).
...access-list-name]
Step 8
match tag tag value [...tag-value]
Match the specified tag value in a list of one or more route tag values.
Each can be an integer from 0 to 4294967295.
Step 9
match interface type number [...type
number]
Match the specified next hop route out one of the specified interfaces.
Step 10
match ip route-source
{access-list-number | access-list-name}
[...access-list-number |
...access-list-name]
Match the address specified by the specified advertised access lists.
Step 11
match route-type {local | internal |
external [type-1 | type-2]}
Match the specified route-type:
Match the specified route metric. The metric-value can be an EIGRP
metric with a specified value from 0 to 4294967295.
•
local—Locally generated BGP routes.
•
internal—OSPF intra-area and interarea routes or EIGRP internal
routes.
•
external—OSPF external routes (Type 1 or Type 2) or EIGRP
external routes.
Step 12
set dampening halflife reuse suppress
max-suppress-time
Set BGP route dampening factors.
Step 13
set local-preference value
Assign a value to a local BGP path.
Step 14
set origin {igp | egp as | incomplete}
Set the BGP origin code.
Step 15
set as-path {tag | prepend
as-path-string}
Modify the BGP autonomous system path.
Step 16
set level {level-1 | level-2 | level-1-2 |
stub-area | backbone}
Set the level for routes that are advertised into the specified area of the
routing domain. The stub-area and backbone are OSPF NSSA and
backbone areas.
Step 17
set metric metric value
Set the metric value to give the redistributed routes (for EIGRP only).
The metric value is an integer from -294967295 to 294967295.
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Step 18
Command
Purpose
set metric bandwidth delay reliability
loading mtu
Set the metric value to give the redistributed routes (for EIGRP only):
•
bandwidth—Metric value or IGRP bandwidth of the route in
kilobits per second in the range 0 to 4294967295
•
delay—Route delay in tens of microseconds in the range 0 to
4294967295.
•
reliability—Likelihood of successful packet transmission expressed
as a number between 0 and 255, where 255 means 100 percent
reliability and 0 means no reliability.
•
loading— Effective bandwidth of the route expressed as a number
from 0 to 255 (255 is 100 percent loading).
•
mtu—Minimum maximum transmission unit (MTU) size of the
route in bytes in the range 0 to 4294967295.
Step 19
set metric-type {type-1 | type-2}
Set the OSPF external metric type for redistributed routes.
Step 20
set metric-type internal
Set the multi-exit discriminator (MED) value on prefixes advertised to
external BGP neighbor to match the IGP metric of the next hop.
Step 21
set weight
Set the BGP weight for the routing table. The value can be from 1 to
65535.
Step 22
end
Return to privileged EXEC mode.
Step 23
show route-map
Display all route maps configured or only the one specified to verify
configuration.
Step 24
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To delete an entry, use the no route-map map tag global configuration command or the no match or no
set route-map configuration commands.
You can distribute routes from one routing domain into another and control route distribution.
Beginning in privileged EXEC mode, follow these steps to control route redistribution. Note that the
keywords are the same as defined in the previous procedure.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router {bgp | rip | ospf | eigrp}
Enter router configuration mode.
Step 3
redistribute protocol [process-id] {level-1 | level-1-2 |
level-2} [metric metric-value] [metric-type type-value]
[match internal | external type-value] [tag tag-value]
[route-map map-tag] [weight weight] [subnets]
Redistribute routes from one routing protocol to
another routing protocol. If no route-maps are
specified, all routes are redistributed. If the keyword
route-map is specified with no map-tag, no routes are
distributed.
Step 4
default-metric number
Cause the current routing protocol to use the same
metric value for all redistributed routes (BGP, RIP and
OSPF).
Step 5
default-metric bandwidth delay reliability loading mtu
Cause the EIGRP routing protocol to use the same
metric value for all non-EIGRP redistributed routes.
Step 6
end
Return to privileged EXEC mode.
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Command
Purpose
Step 7
show route-map
Display all route maps configured or only the one
specified to verify configuration.
Step 8
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To disable redistribution, use the no form of the commands.
The metrics of one routing protocol do not necessarily translate into the metrics of another. For example,
the RIP metric is a hop count, and the IGRP metric is a combination of five qualities. In these situations,
an artificial metric is assigned to the redistributed route. Uncontrolled exchanging of routing information
between different routing protocols can create routing loops and seriously degrade network operation.
If you have not defined a default redistribution metric that replaces metric conversion, some automatic
metric translations occur between routing protocols:
•
RIP can automatically redistribute static routes. It assigns static routes a metric of 1 (directly
connected).
•
Any protocol can redistribute other routing protocols if a default mode is in effect.
Configuring Policy-Based Routing
You can use policy-based routing (PBR) to configure a defined policy for traffic flows. By using PBR,
you can have more control over routing by reducing the reliance on routes derived from routing
protocols. PBR can specify and implement routing policies that allow or deny paths based on:
•
Identity of a particular end system
•
Application
•
Protocol
You can use PBR to provide equal-access and source-sensitive routing, routing based on interactive
versus batch traffic, or routing based on dedicated links. For example, you could transfer stock records
to a corporate office on a high-bandwidth, high-cost link for a short time while transmitting routine
application data such as e-mail over a low-bandwidth, low-cost link.
With PBR, you classify traffic using access control lists (ACLs) and then make traffic go through a
different path. PBR is applied to incoming packets. All packets received on an interface with PBR
enabled are passed through route maps. Based on the criteria defined in the route maps, packets are
forwarded (routed) to the appropriate next hop.
•
If packets do not match any route map statements, all set clauses are applied.
•
If a statement is marked as permit and the packets do not match any route-map statements, the
packets are sent through the normal forwarding channels, and destination-based routing is
performed.
•
For PBR, route-map statements marked as deny are not supported.
For more information about configuring route maps, see the “Using Route Maps to Redistribute Routing
Information” section on page 41-94.
You can use standard IP ACLs to specify match criteria for a source address or extended IP ACLs to
specify match criteria based on an application, a protocol type, or an end station. The process proceeds
through the route map until a match is found. If no match is found, normal destination-based routing
occurs. There is an implicit deny at the end of the list of match statements.
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If match clauses are satisfied, you can use a set clause to specify the IP addresses identifying the next
hop router in the path.
For details about PBR commands and keywords, see the Cisco IOS IP Command Reference, Volume 2 of
3: Routing Protocols, Release 12.2. For a list of PBR commands that are visible but not supported by the
switch, see Appendix C, “Unsupported Commands in Cisco IOS Release 12.2(55)SE.”
PBR configuration is applied to the whole stack, and all switches use the stack master configuration.
Note
This software release does not support Policy-Based Routing (PBR) when processing IPv4 and IPv6
traffic.
PBR Configuration Guidelines
•
To use PBR, you must have the IP services feature set enabled on the switch or stack master.
•
Multicast traffic is not policy-routed. PBR applies to only to unicast traffic.
•
You can enable PBR on a routed port or an SVI.
•
The switch does not support route-map deny statements for PBR.
•
You can apply a policy route map to an EtherChannel port channel in Layer 3 mode, but you cannot
apply a policy route map to a physical interface that is a member of the EtherChannel. If you try to
do so, the command is rejected. When a policy route map is applied to a physical interface, that
interface cannot become a member of an EtherChannel.
•
You can define a maximum of 246 IP policy route maps on the switch or switch stack.
•
You can define a maximum of 512 access control entries (ACEs) for PBR on the switch or switch
stack.
•
When configuring match criteria in a route map, follow these guidelines:
– Do not match ACLs that permit packets destined for a local address. PBR would forward these
packets, which could cause ping or Telnet failure or route protocol flappping.
– Do not match ACLs with deny ACEs. Packets that match a deny ACE are sent to the CPU, which
could cause high CPU utilization.
•
To use PBR, you must first enable the routing template by using the sdm prefer routing global
configuration command. PBR is not supported with the VLAN or default template. For more
information on the SDM templates, see Chapter 8, “Configuring SDM Templates.”
•
VRF and PBR are mutually exclusive on a switch interface. You cannot enable VRF when PBR is
enabled on an interface. The reverse is also true, you cannot enable PBR when VRF is enabled on
an interface.
•
Web Cache Communication Protocol (WCCP) and PBR are mutually exclusive on a switch
interface. You cannot enable WCCP when PBR is enabled on an interface. The reverse is also true,
you cannot enable PBR when WCCP is enabled on an interface.
•
The number of hardware entries used by PBR depends on the route map itself, the ACLs used, and
the order of the ACLs and route-map entries.
•
Policy-based routing based on packet length, TOS, set interface, set default next hop, or set default
interface are not supported. Policy maps with no valid set actions or with set action set to Don’t
Fragment are not supported.
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•
The switch supports QoS DSCP and IP precedence matching in PBR route maps, with these
limitations:
– You cannot apply QoS DSCP mutation maps and PBR route maps to the same interface.
– You cannot configure DSCP transparency and PBR DSCP route maps on the same switch.
– When you configure PBR with QoS DSCP, you can set QoS to be enabled (by entering the mls
qos global configuration command) or disabled (by entering the no mls qos command). When
QoS is enabled, to ensure that the DSCP value of the traffic is unchanged, you should configure
DSCP trust state on the port where traffic enters the switch by entering the mls qos trust dscp
interface configuration command. If the trust state is not DSCP, by default all nontrusted traffic
would have the DSCP value marked as 0.
Enabling PBR
By default, PBR is disabled on the switch. To enable PBR, you must create a route map that specifies
the match criteria and the resulting action if all of the match clauses are met. Then, you must enable PBR
for that route map on an interface. All packets arriving on the specified interface matching the match
clauses are subject to PBR.
PBR can be fast-switched or implemented at speeds that do not slow down the switch. Fast-switched
PBR supports most match and set commands. PBR must be enabled before you enable fast-switched
PBR. Fast-switched PBR is disabled by default.
Packets that are generated by the switch, or local packets, are not normally policy-routed. When you
globally enable local PBR on the switch, all packets that originate on the switch are subject to local PBR.
Local PBR is disabled by default.
Note
To enable PBR, the switch or stack master must be running the IP services feature set.
Beginning in privileged EXEC mode, follow these steps to configure PBR:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
route-map map-tag [permit] [sequence number]
Define any route maps used to control where packets are
output, and enter route-map configuration mode.
•
map-tag—A meaningful name for the route map. The ip
policy route-map interface configuration command uses
this name to reference the route map. Multiple route maps
might share the same map tag name.
•
(Optional) If permit is specified and the match criteria
are met for this route map, the route is policy-routed as
controlled by the set actions.
Note
•
The route-map deny statement is not supported in
PBR route maps to be applied to an interface.
sequence number (Optional)— Number that shows the
position of a new route map in the list of route maps
already configured with the same name.
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Step 3
Command
Purpose
match ip address {access-list-number |
access-list-name} [...access-list-number |
...access-list-name]
Match the source and destination IP address that is permitted
by one or more standard or extended access lists.
Note
Do not enter an ACL with a deny ACE or an ACL that
permits a packet destined for a local address.
If you do not specify a match command, the route map applies
to all packets.
Step 4
set ip next-hop ip-address [...ip-address]
Specify the action to take on the packets that match the
criteria. Set next hop to which to route the packet (the next hop
must be adjacent).
Step 5
exit
Return to global configuration mode.
Step 6
interface interface-id
Enter interface configuration mode, and specify the interface
to configure.
Step 7
ip policy route-map map-tag
Enable PBR on a Layer 3 interface, and identify the route map
to use. You can configure only one route map on an interface.
However, you can have multiple route map entries with
different sequence numbers. These entries are evaluated in
sequence number order until the first match. If there is no
match, packets are routed as usual.
Note
If the IP policy route map contains a deny statement,
the configuration fails.
Step 8
ip route-cache policy
(Optional) Enable fast-switching PBR. You must first enable
PBR before enabling fast-switching PBR.
Step 9
exit
Return to global configuration mode.
Step 10
ip local policy route-map map-tag
(Optional) Enable local PBR to perform policy-based routing
on packets originating at the switch. This applies to packets
generated by the switch and not to incoming packets.
Step 11
end
Return to privileged EXEC mode.
Step 12
show route-map [map-name]
(Optional) Display all route maps configured or only the one
specified to verify configuration.
Step 13
show ip policy
(Optional) Display policy route maps attached to interfaces.
Step 14
show ip local policy
(Optional) Display whether or not local policy routing is
enabled and, if so, the route map being used.
Step 15
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no route-map map-tag global configuration command or the no match or no set route-map
configuration commands to delete an entry. Use the no ip policy route-map map-tag interface
configuration command to disable PBR on an interface. Use the no ip route-cache policy interface
configuration command to disable fast-switching PBR. Use the no ip local policy route-map map-tag
global configuration command to disable policy-based routing on packets originating on the switch.
Filtering Routing Information
You can filter routing protocol information by performing the tasks described in this section.
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Note
When routes are redistributed between OSPF processes, no OSPF metrics are preserved.
Setting Passive Interfaces
To prevent other routers on a local network from dynamically learning about routes, you can use the
passive-interface router configuration command to keep routing update messages from being sent
through a router interface. When you use this command in the OSPF protocol, the interface address you
specify as passive appears as a stub network in the OSPF domain. OSPF routing information is neither
sent nor received through the specified router interface.
In networks with many interfaces, to avoid having to manually set them as passive, you can set all
interfaces to be passive by default by using the passive-interface default router configuration command
and manually setting interfaces where adjacencies are desired.
Beginning in privileged EXEC mode, follow these steps to configure passive interfaces:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router {bgp | rip | ospf | eigrp}
Enter router configuration mode.
Step 3
passive-interface interface-id
Suppress sending routing updates through the specified Layer 3
interface.
Step 4
passive-interface default
(Optional) Set all interfaces as passive by default.
Step 5
no passive-interface interface type
(Optional) Activate only those interfaces that need to have
adjacencies sent.
Step 6
network network-address
(Optional) Specify the list of networks for the routing process. The
network-address is an IP address.
Step 7
end
Return to privileged EXEC mode.
Step 8
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use a network monitoring privileged EXEC command such as show ip ospf interface to verify the
interfaces that you enabled as passive, or use the show ip interface privileged EXEC command to verify
the interfaces that you enabled as active.
To re-enable the sending of routing updates, use the no passive-interface interface-id router
configuration command. The default keyword sets all interfaces as passive by default. You can then
configure individual interfaces where you want adjacencies by using the no passive-interface router
configuration command. The default keyword is useful in Internet service provider and large enterprise
networks where many of the distribution routers have more than 200 interfaces.
Controlling Advertising and Processing in Routing Updates
You can use the distribute-list router configuration command with access control lists to suppress routes
from being advertised in routing updates and to prevent other routers from learning one or more routes.
When used in OSPF, this feature applies to only external routes, and you cannot specify an interface
name.
You can also use a distribute-list router configuration command to avoid processing certain routes listed
in incoming updates. (This feature does not apply to OSPF.)
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Configuring Protocol-Independent Features
Beginning in privileged EXEC mode, follow these steps to control the advertising or processing of
routing updates:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router {bgp | rip | eigrp}
Enter router configuration mode.
Step 3
distribute-list {access-list-number |
access-list-name} out [interface-name | routing
process | autonomous-system-number]
Permit or deny routes from being advertised in routing
updates, depending upon the action listed in the access list.
Step 4
distribute-list {access-list-number |
access-list-name} in [type-number]
Suppress processing in routes listed in updates.
Step 5
end
Return to privileged EXEC mode.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
Use the no distribute-list in router configuration command to change or cancel a filter. To cancel
suppression of network advertisements in updates, use the no distribute-list out router configuration
command.
Filtering Sources of Routing Information
Because some routing information might be more accurate than others, you can use filtering to prioritize
information coming from different sources. An administrative distance is a rating of the trustworthiness
of a routing information source, such as a router or group of routers. In a large network, some routing
protocols can be more reliable than others. By specifying administrative distance values, you enable the
router to intelligently discriminate between sources of routing information. The router always picks the
route whose routing protocol has the lowest administrative distance. Table 41-16 on page 41-93 shows
the default administrative distances for various routing information sources.
Because each network has its own requirements, there are no general guidelines for assigning
administrative distances.
Beginning in privileged EXEC mode, follow these steps to filter sources of routing information:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
router {bgp | rip | ospf | eigrp}
Enter router configuration mode.
Step 3
distance weight {ip-address {ip-address mask}}
[ip access list]
Define an administrative distance.
weight—The administrative distance as an integer from
10 to 255. Used alone, weight specifies a default
administrative distance that is used when no other
specification exists for a routing information source.
Routes with a distance of 255 are not installed in the
routing table.
(Optional) ip access list—An IP standard or extended
access list to be applied to incoming routing updates.
Step 4
end
Return to privileged EXEC mode.
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Command
Purpose
Step 5
show ip protocols
Display the default administrative distance for a
specified routing process.
Step 6
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To remove a distance definition, use the no distance router configuration command.
Managing Authentication Keys
Key management is a method of controlling authentication keys used by routing protocols. Not all
protocols can use key management. Authentication keys are available for EIGRP and RIP Version 2.
Before you manage authentication keys, you must enable authentication. See the appropriate protocol
section to see how to enable authentication for that protocol. To manage authentication keys, define a
key chain, identify the keys that belong to the key chain, and specify how long each key is valid. Each
key has its own key identifier (specified with the key number key chain configuration command), which
is stored locally. The combination of the key identifier and the interface associated with the message
uniquely identifies the authentication algorithm and Message Digest 5 (MD5) authentication key in use.
You can configure multiple keys with life times. Only one authentication packet is sent, regardless of
how many valid keys exist. The software examines the key numbers in order from lowest to highest, and
uses the first valid key it encounters. The lifetimes allow for overlap during key changes. Note that the
router must know these lifetimes.
Beginning in privileged EXEC mode, follow these steps to manage authentication keys:
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
key chain name-of-chain
Identify a key chain, and enter key chain configuration
mode.
Step 3
key number
Identify the key number. The range is 0 to 2147483647.
Step 4
key-string text
Identify the key string. The string can contain from 1 to
80 uppercase and lowercase alphanumeric characters,
but the first character cannot be a number.
Step 5
accept-lifetime start-time {infinite | end-time | duration
seconds}
(Optional) Specify the time period during which the key
can be received.
The start-time and end-time syntax can be either
hh:mm:ss Month date year or hh:mm:ss date Month
year. The default is forever with the default start-time
and the earliest acceptable date as January 1, 1993. The
default end-time and duration is infinite.
Step 6
send-lifetime start-time {infinite | end-time | duration
seconds}
(Optional) Specify the time period during which the key
can be sent.
The start-time and end-time syntax can be either
hh:mm:ss Month date year or hh:mm:ss date Month
year. The default is forever with the default start-time
and the earliest acceptable date as January 1, 1993. The
default end-time and duration is infinite.
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Monitoring and Maintaining the IP Network
Command
Purpose
Step 7
end
Return to privileged EXEC mode.
Step 8
show key chain
Display authentication key information.
Step 9
copy running-config startup-config
(Optional) Save your entries in the configuration file.
To remove the key chain, use the no key chain name-of-chain global configuration command.
Monitoring and Maintaining the IP Network
You can remove all contents of a particular cache, table, or database. You can also display specific
statistics. Use the privileged EXEC commands in Table 41-17 to clear routes or display status:
Table 41-17
Commands to Clear IP Routes or Display Route Status
Command
Purpose
clear ip route {network [mask | *]}
Clear one or more routes from the IP routing table.
show ip protocols
Display the parameters and state of the active routing protocol
process.
show ip route [address [mask] [longer-prefixes]] |
[protocol [process-id]]
Display the current state of the routing table.
show ip route summary
Display the current state of the routing table in summary form.
show ip route supernets-only
Display supernets.
show ip cache
Display the routing table used to switch IP traffic.
show route-map [map-name]
Display all route maps configured or only the one specified.
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Monitoring and Maintaining the IP Network
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