Configuring the Cisco IOS DHCP Relay Agent

Configuring the Cisco IOS DHCP Relay Agent
Last Updated: July 27, 2012
All Cisco routers that run Cisco software include a DHCP server and the relay agent software. A DHCP
relay agent is any host or IP router that forwards DHCP packets between clients and servers. This module
describes the concepts and tasks needed to configure the Cisco IOS DHCP relay agent.
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Finding Feature Information, page 1
Prerequisites for Configuring the Cisco IOS DHCP Relay Agent, page 1
Information About the DHCP Relay Agent, page 2
How to Configure the DHCP Relay Agent, page 2
Configuration Examples for the Cisco IOS DHCP Relay Agent, page 24
Additional References, page 27
Technical Assistance, page 28
Feature Information for the Cisco IOS DHCP Relay Agent, page 29
Glossary, page 35
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats
and feature information, see Bug Search Tool and the release notes for your platform and software release.
To find information about the features documented in this module, and to see a list of the releases in which
each feature is supported, see the feature information table at the end of this module.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support.
To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Prerequisites for Configuring the Cisco IOS DHCP Relay
Agent
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Before you configure the DHCP relay agent, you should understand the concepts documented in the
“DHCP Overview” module.
The Cisco IOS DHCP server and relay agent are enabled by default. You can verify whether they have
been disabled by checking your configuration file. If they have been disabled, the no service dhcp
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DHCP Relay Agent Overview
Information About the DHCP Relay Agent
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command will appear in the configuration file. Use the service dhcp command to reenable the
functionality if necessary.
The Cisco IOS DHCP relay agent will be enabled on an interface only when the ip helper-address
command is configured. This command enables the DHCP broadcast to be forwarded to the
configured DHCP server.
Information About the DHCP Relay Agent
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DHCP Relay Agent Overview, page 2
DHCP Relay Agent Overview
A DHCP relay agent is any host that forwards DHCP packets between clients and servers. Relay agents
forward requests and replies between clients and servers when they are not on the same physical subnet.
Relay agent forwarding is distinct from the normal forwarding of an IP router, where IP datagrams are
switched between networks somewhat transparently. In contrast, when relay agents receive DHCP
messages, the agents generate a new DHCP message to send out through another interface. The relay agent
sets the gateway IP address (the giaddr field of the DHCP packet) and, if configured, adds the relay agent
information option (option 82) to the packet and forwards the packet to the DHCP server. The reply from
the server is forwarded back to the client after removing option 82.
The Cisco IOS DHCP relay agent supports the use of unnumbered interfaces, including the use of smart
relay agent forwarding. For DHCP clients that are connected though unnumbered interfaces, the DHCP
relay agent automatically adds a static host route after the DHCP client obtains an address, specifying the
unnumbered interface as the outbound interface. The route is automatically removed once the lease time
expires or when the client releases the address.
How to Configure the DHCP Relay Agent
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Specifying the Packet Forwarding Address, page 2
Configuring Support for the Relay Agent Information Option, page 5
Configuring Per-Interface Support for the Relay Agent Information Option, page 8
Configuring the Subscriber Identifier Suboption of the Relay Agent Information Option, page 11
Configuring DHCP Relay Class Support for Client Identification, page 12
Configuring DHCP Relay Agent Support for MPLS VPNs, page 15
Configuring Support for Relay Agent Information Option Encapsulation, page 18
Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay
Agent Forwarding, page 21
Configuring Support for Private and Standard Suboption Numbers, page 22
Troubleshooting the DHCP Relay Agent, page 23
Specifying the Packet Forwarding Address
DHCP clients need to use UDP broadcasts to send their initial DHCPDISCOVER messages because the
clients do not have information about the network to which they are attached. If the client is on a network
segment that does not include a server, UDP broadcasts are not usually forwarded because most routers are
2
Specifying the Packet Forwarding Address
How to Configure the DHCP Relay Agent
configured to not forward broadcast traffic. When the DHCP client broadcasts a DHCPDISCOVER
message, the relay agent sends the broadcast message toward the server, which may create Address
Resolution Protocol (ARP) entries due to unnecessary ARP checks performed by the client after receiving
the ACK message. If there are two entries in the ARP table, one times out after the ARP timeout. You can
remedy this situation by configuring the interface of your router that is receiving the broadcasts to forward
certain classes of broadcasts to a helper address. You can use more than one helper address per interface.
When a router forwards these address assignment/parameter requests, it acts as a DHCP relay agent. The
Cisco router implementation of the DHCP relay agent is provided through the ip helper-address interface
configuration command.
In the figure below, the DHCP client broadcasts a request for an IP address and additional configuration
parameters on its local LAN. Router B, acting as a DHCP relay agent, picks up the broadcast and generates
a new DHCP message to send out on another interface. As part of this DHCP message, the relay agent
inserts the IP address of the interface containing the ip helper-address command into the gateway IP
address (giaddr) field of the DHCP packet. This IP address enables the DHCP server to determine which
subnet should receive the packet. The DHCP relay agent sends the local broadcast, through IP unicast, to
the DHCP server address 172.16.1.2 that is specified by the ip helper-address interface configuration
command.
Forwarding UDP Broadcasts to a DHCP Server Using a Helper Address
DHCP client
DHCP server
172.16.1.2
172.16.1.1
Router A
172.31.1.1
ip helper-address 172.16.1.2
Router B
127132
Figure 1
Perform this task to configure the DHCP relay agent to forward packets to a DHCP server.
SUMMARY STEPS
1. enable
2. configure terminal
3. interface type number
4. ip helper-address address
5. exit
6. ip dhcp relay prefer known-good-server
7. exit
3
Specifying the Packet Forwarding Address
How to Configure the DHCP Relay Agent
DETAILED STEPS
Command or Action
Step 1 enable
Purpose
Enables privileged EXEC mode.
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Enter your password if prompted.
Example:
Device> enable
Step 2 configure terminal
Enters global configuration mode.
Example:
Device# configure terminal
Step 3 interface type number
Configures an interface and enters interface configuration mode.
Example:
Device(config)# interface
FastEthernet0/0
Step 4 ip helper-address address
Forwards UDP broadcasts, including BOOTP and DHCP.
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Example:
Device(config-if)# ip helper-address
172.16.1.2
Step 5 exit
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The address argument can be a specific DHCP server address, or it
can be the network address if other DHCP servers are on the
destination network segment. The network address enables other
servers to respond to DHCP requests.
If you have multiple servers, you can configure one helper address
for each server.
Exits interface configuration mode and returns to global configuration
mode.
Example:
Device(config-if)# exit
Step 6 ip dhcp relay prefer known-good-server
Example:
Device(config)# ip dhcp relay prefer
known-good-server
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(Optional) Reduces the frequency with which the DHCP clients change
their addresses and forwards client requests to the server that handled the
previous request.
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The DHCP relay agent deletes the ARP entries for addresses offered
to the DHCP client on unnumbered interfaces.
Configuring Support for the Relay Agent Information Option
How to Configure the DHCP Relay Agent
Command or Action
Step 7 exit
Purpose
Returns to privileged EXEC mode.
Example:
Device(config)# exit
Configuring Support for the Relay Agent Information Option
Automatic DHCP address allocation is typically based on an IP address, which may be either the gateway
IP address (giaddr field of the DHCP packet) or the incoming interface IP address. In some networks,
additional information may be required to further determine the IP addresses that need to be allocated. By
using the relay agent information option (option 82), the Cisco IOS relay agent can include additional
information about itself when forwarding client-originated DHCP packets to a DHCP server. Cisco
software supports this functionality by using the ip dhcp relay information option command. The relay
agent will automatically add the circuit identifier suboption and the remote ID suboption to the relay agent
information option and forward them to the DHCP server.
The DHCP server can use this information to assign IP addresses, perform access control, and set quality of
service (QoS) and security policies (or other parameter-assignment policies) for each subscriber of a
service provider network.
The figure below shows how the relay agent information option is inserted into the DHCP packet as
follows:
1 The DHCP client generates a DHCP request and broadcasts it on the network.
2 The DHCP relay agent intercepts the broadcast DHCP request packet and inserts the relay agent
information option (option 82) into the packet. The relay agent information option contains related
suboptions.
3 The DHCP relay agent unicasts the DHCP packet to the DHCP server.
4 The DHCP server receives the packet, uses the suboptions to assign IP addresses and other
configuration parameters to the packet, and forwards the packet back to the client.
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Configuring Support for the Relay Agent Information Option
How to Configure the DHCP Relay Agent
5 The suboption fields are stripped off of the packet by the relay agent while forwarding the packet to the
client.
Figure 2
Operation of the Relay Agent Information Option
Clients broadcast
for DHCP requests
Option 82
Append remote ID + circuit ID
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2
DHCP server
If Option 82 aware, use
appended information
3
ip helper-address command
Takes DHCP requests and
unicasts to DHCP server
DHCP server
5
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Strip-off option 82,
implement policy and forward
IP address assignment
Based on appended information,
return IP address and policies
DHCP client
127133
DHCP client
A DHCP relay agent may receive a message from another DHCP relay agent that already contains relay
information. By default, the relay information from the previous relay agent is replaced. If this behavior is
not suitable for your network, you can use the ip dhcp relay information policy {drop | keep | replace}
global configuration command to change it.
To ensure the correct operation of the reforwarding policy, disable the relay agent information check by
using the no ip dhcp relay information check global configuration command.
It is important to understand how DHCP options work. See the “DHCP Overview” module for more
information.
Note
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If the ip dhcp relay information command is configured in global configuration mode but not
configured in interface configuration mode, the global configuration is applied to all interfaces.
If the ip dhcp relay information command is configured in both global configuration mode and
interface configuration mode, the interface configuration command takes precedence over the global
configuration command. However, the global configuration is applied to interfaces without the
interface configuration.
If the ip dhcp relay information command is not configured in global configuration mode but is
configured in interface configuration mode, only the interface with the configuration option applied is
affected. All other interfaces are not impacted by the configuration.
See the “Configuring Relay Agent Information Option Support per Interface” section for more information
on per-interface support for the relay agent information option.
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Configuring Support for the Relay Agent Information Option
How to Configure the DHCP Relay Agent
SUMMARY STEPS
1. enable
2. configure terminal
3. ip dhcp relay information option
4. ip dhcp relay information check
5. ip dhcp relay information policy {drop | keep | replace}
6. ip dhcp relay information trust-all
7. end
8. show ip dhcp relay information trusted-sources
DETAILED STEPS
Command or Action
Step 1 enable
Purpose
Enables privileged EXEC mode.
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Enter your password if prompted.
Example:
Device> enable
Step 2 configure terminal
Enters global configuration mode.
Example:
Device# configure terminal
Step 3 ip dhcp relay information option
Example:
Enables the system to insert the DHCP relay agent information option
(option-82 field) in BOOTREQUEST messages forwarded to a DHCP server.
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This function is disabled by default.
Device(config)# ip dhcp relay
information option
Step 4 ip dhcp relay information check
Example:
Device(config)# ip dhcp relay
information check
(Optional) Configures DHCP to check whether the relay agent information
option in forwarded BOOTREPLY messages is valid.
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By default, DHCP verifies whether the option-82 field in DHCP reply
packets that it receives from the DHCP server is valid. If an invalid
message is received, the relay agent drops the packet. If a valid message
is received, the relay agent removes the option-82 field and forwards the
packet. Use the ip dhcp relay information check command to reenable
this functionality if it has been disabled.
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Configuring Per-Interface Support for the Relay Agent Information Option
How to Configure the DHCP Relay Agent
Command or Action
Purpose
Step 5 ip dhcp relay information policy {drop (Optional) Configures the reforwarding policy (that specifies what a relay
agent should do if a message already contains relay information) for a DHCP
| keep | replace}
relay agent.
Example:
Device(config)# ip dhcp relay
information policy replace
Step 6 ip dhcp relay information trust-all
(Optional) Configures all interfaces on a router as trusted sources of the
DHCP relay information option.
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Example:
Device(config)# ip dhcp relay
information trust-all
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Step 7 end
By default, if the gateway address is set to all zeros in the DHCP packet
and the relay agent information option is already present in the packet,
the DHCP relay agent will discard the packet. Use the ip dhcp relay
information trust-all command to override this behavior and accept the
packets.
This command is useful if there is a switch placed between the client and
the relay agent that may insert option 82. Use this command to ensure
that these packets do not get dropped.
You can configure an individual interface as a trusted source of the
DHCP relay information option by using the ip dhcp relay information
trusted interface configuration mode command.
Returns to privileged EXEC mode.
Example:
Device(config)# end
Step 8 show ip dhcp relay information
trusted-sources
(Optional) Displays all interfaces that are configured to be a trusted source for
the DHCP relay information option.
Example:
Device# show ip dhcp relay
information trusted-sources
Configuring Per-Interface Support for the Relay Agent Information Option
The interface configuration allows a Cisco router to reach subscribers with different DHCP option 82
requirements on different interfaces.
It is important to understand how DHCP options work. See the “DHCP Overview” module for more
information.
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Configuring Per-Interface Support for the Relay Agent Information Option
How to Configure the DHCP Relay Agent
Note
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If the ip dhcp relay information command is configured in global configuration mode but not
configured in interface configuration mode, the global configuration is applied to all interfaces.
If the ip dhcp relay information command is configured in both global configuration mode and
interface configuration mode, the interface configuration command takes precedence over the global
configuration command. However, the global configuration is applied to interfaces without the
interface configuration.
If the ip dhcp relay information command is not configured in global configuration mode but is
configured in interface configuration mode, only the interface on which the configuration option is
applied is affected. All other interfaces are not impacted by the configuration.
SUMMARY STEPS
1. enable
2. configure terminal
3. interface type number
4. ip dhcp relay information option-insert [none]
5. ip dhcp relay information check-reply [none]
6. ip dhcp relay information policy-action {drop | keep | replace}
7. exit
8. Repeat Steps 3 through 7 to configure relay agent information settings on different interfaces.
DETAILED STEPS
Command or Action
Step 1 enable
Purpose
Enables privileged EXEC mode.
•
Enter your password if prompted.
Example:
Device> enable
Step 2 configure terminal
Enters global configuration mode.
Example:
Device# configure terminal
Step 3 interface type number
Configures an interface and enters interface configuration mode.
Example:
Device(config)# interface
FastEthernet0/0
9
Configuring Per-Interface Support for the Relay Agent Information Option
How to Configure the DHCP Relay Agent
Command or Action
Step 4 ip dhcp relay information option-insert
[none]
Example:
Purpose
Enables the system to insert the DHCP relay agent information option
(option-82 field) in forwarded BOOTREQUEST messages to a DHCP
server.
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Device(config-if)# ip dhcp relay
information option-insert
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Step 5 ip dhcp relay information check-reply
[none]
Configures a DHCP server to validate the relay information option in
forwarded BOOTREPLY messages.
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Example:
Device(config-if)# ip dhcp relay
information check-reply
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Step 6 ip dhcp relay information policy-action
{drop | keep | replace}
This function is disabled by default. However, if support for the relay
agent information option is configured in global configuration mode,
but not configured in interface configuration mode, the interface
inherits the global configuration.
The ip dhcp relay information option-insert none interface
configuration command is saved in the running configuration. This
command takes precedence over any global relay agent information
configuration.
By default, DHCP verifies whether the option-82 field in the DHCP
reply packets that it receives from the DHCP server is valid. If an
invalid message is received, the relay agent drops the packet. If a valid
message is received, the relay agent removes the option-82 field and
forwards the packet. Use the ip dhcp relay information check-reply
command to reenable this functionality if it has been disabled.
The ip dhcp relay information check-reply none interface
configuration command option is saved in the running configuration.
This command takes precedence over any global relay agent
information configuration.
Configures the information reforwarding policy (that specifies what a relay
agent should do if a message already contains relay information) for a DHCP
relay agent.
Example:
Device(config-if)# ip dhcp relay
information policy-action replace
Step 7 exit
Exits interface configuration mode.
Example:
Device(config-if)# exit
Step 8 Repeat Steps 3 through 7 to configure relay —
agent information settings on different
interfaces.
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Configuring the Subscriber Identifier Suboption of the Relay Agent Information Option
How to Configure the DHCP Relay Agent
Configuring the Subscriber Identifier Suboption of the Relay Agent
Information Option
Perform this task to enable an ISP to add a unique identifier to the subscriber identifier suboption of the
relay agent information option. The unique identifier enables an ISP to identify a subscriber, assign specific
actions to that subscriber (for example, assignment of the host IP address, subnet mask, and domain name
system [DNS]), and trigger accounting.
Before the introduction of the subscriber identifier suboption, if a subscriber moved from one Network
Access Server to another, each ISP had to be informed of the change and all ISPs had to reconfigure the
DHCP settings for the affected customers at the same time. Even if the service was not changed, every
move involved administrative changes in the ISP environment. With the introduction of the subscriber
identifier suboption, if a subscriber moves from one Network Access Server to another, there is no need for
a change in the configuration on the DHCP server or the ISPs.
You should configure a unique identifier for each subscriber.
The new configurable subscriber identifier suboption should be configured on the interface that is
connected to the client. When a subscriber moves from one Network Access Server to another, the interface
configuration should also be changed.
The server should be able to recognize the new suboption.
SUMMARY STEPS
1. enable
2. configure terminal
3. ip dhcp relay information option
4. interface type number
5. ip dhcp relay information option subscriber-id string
6. end
DETAILED STEPS
Command or Action
Step 1 enable
Purpose
Enables privileged EXEC mode.
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Enter your password if prompted.
Example:
Device> enable
Step 2 configure terminal
Enters global configuration mode.
Example:
Device# configure terminal
11
Configuring DHCP Relay Class Support for Client Identification
How to Configure the DHCP Relay Agent
Command or Action
Step 3 ip dhcp relay information option
Example:
Purpose
Enables the system to insert the DHCP relay agent information
option (option-82 field) in forwarded BOOTREQUEST
messages to a DHCP server.
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This function is disabled by default.
Router(config)# ip dhcp relay information
option
Step 4 interface type number
Configures an interface and enters interface configuration mode.
Example:
Device(config)# interface atm4/0.1
Step 5 ip dhcp relay information option subscriber-id
string
Specifies that a DHCP relay agent add a subscriber identifier
suboption to the relay information option.
Note The ip dhcp relay information option subscriber-id
command is disabled by default to ensure backward
capability.
Example:
Device(config-if)# ip dhcp relay information
option subscriber-id newsubscriber123
Step 6 end
Returns to privileged EXEC mode.
Example:
Device(config-if)# end
Configuring DHCP Relay Class Support for Client Identification
DHCP relay class support for client identification allows the Cisco relay agent to forward client-generated
DHCP messages to different DHCP servers based on the content of the following four options:
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Option 60: vendor class identifier
Option 77: user class
Option 124: vendor-identifying vendor class
Option 125: vendor-identifying vendor-specific information
Each option identifies the type of client that is sending the DHCP message.
Relay pools provide a method to define DHCP pools that are not used for address allocation. These relay
pools can specify that DHCP messages from clients on a specific subnet should be forwarded to a specific
DHCP server. These relay pools can be configured with relay classes inside the pool that help determine
the forwarding behavior.
For example, after receiving the option in a DHCP DISCOVER message, the relay agent will match and
identify the relay class from the relay pool and then direct the DHCP DISCOVER message to the DHCP
server associated with that identified relay class.
12
Configuring DHCP Relay Class Support for Client Identification
How to Configure the DHCP Relay Agent
In an example application, a Cisco router acting as a DHCP relay agent receives DHCP requests from two
VoIP services (H.323 and the Session Initiation Protocol [SIP]). The requesting devices are identified by
option 60.
Both VoIP services have a different back-office infrastructure, so they cannot be serviced by the same
DHCP server. Requests for H.323 devices must be forwarded to the H.323 server, and requests from SIP
devices must be forwarded to the SIP server. The solution is to configure the relay agent with relay classes
that are configured to match option 60 values sent by the client devices. Based on the option value, the
relay agent will match and identify the relay class, and forward the DHCP DISCOVER message to the
DHCP server associated with the identified relay class.
The Cisco IOS DHCP server examines the relay classes that are applicable to a pool and then uses the exact
match class regardless of the configuration order. If the exact match is not found, the DHCP server uses the
first default match found.
It is important to understand how DHCP options work. See the “DHCP Overview” module for more
information.
You must know the hexadecimal value of each byte location in the options to be able to configure the
option hex command. The format may vary from product to product. Contact the relay agent vendor for
this information.
SUMMARY STEPS
1. enable
2. configure terminal
3. ip dhcp class class-name
4. option code hex hex-pattern [*][mask bit-mask-pattern]
5. exit
6. Repeat Steps 3 through 5 for each DHCP class that you need to configure.
7. ip dhcp pool name
8. relay source ip-address subnet-mask
9. class class-name
10. relay target [vrf vrf-name | global] ip-address
11. exit
12. Repeat Steps 9 through 11 for each DHCP class that you need to configure.
DETAILED STEPS
Command or Action
Step 1 enable
Purpose
Enables privileged EXEC mode.
•
Enter your password if prompted.
Example:
Device> enable
13
Configuring DHCP Relay Class Support for Client Identification
How to Configure the DHCP Relay Agent
Command or Action
Step 2 configure terminal
Purpose
Enters global configuration mode.
Example:
Device# configure terminal
Step 3 ip dhcp class class-name
Defines a DHCP class and enters DHCP class
configuration mode.
Example:
Device(config)# ip dhcp class SIP
Step 4 option code hex hex-pattern [*][mask bit-mask-pattern]
Enables the relay agent to make forwarding decisions
based on DHCP options inserted in the DHCP message.
Example:
Device(dhcp-class)# option 60 hex 010203
Step 5 exit
Exits DHCP class configuration mode.
Example:
Device(dhcp-class)# exit
Step 6 Repeat Steps 3 through 5 for each DHCP class that you
need to configure.
—
Step 7 ip dhcp pool name
Configures a DHCP pool on a DHCP server and enters
DHCP pool configuration mode.
Example:
Device(config)# ip dhcp pool ABC
Step 8 relay source ip-address subnet-mask
Configures the relay source.
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Example:
Device(dhcp-config)# relay source 10.2.0.0
255.0.0.0
Step 9 class class-name
Example:
Device(dhcp-config)# class SIP
14
This command is similar to the network command in
a normal DHCP network pool, because it restricts the
use of the address pool to packets arriving on the
interface whose configured IP address and mask
match the relay source configuration.
Associates a class with a DHCP pool and enters DHCP
pool class configuration mode.
Configuring DHCP Relay Agent Support for MPLS VPNs
How to Configure the DHCP Relay Agent
Command or Action
Step 10 relay target [vrf vrf-name | global] ip-address
Purpose
Configures an IP address for a DHCP server to which
packets are forwarded.
Example:
Device(config-dhcp-pool-class)# relay target
10.21.3.1
Step 11 exit
Exits DHCP pool class configuration mode.
Example:
Device(config-dhcp-pool-class)# exit
Step 12 Repeat Steps 9 through 11 for each DHCP class that you
need to configure.
—
Configuring DHCP Relay Agent Support for MPLS VPNs
DHCP relay support for Multiprotocol Label Switching (MPLS) VPNs enables a network administrator to
conserve address space by allowing overlapping addresses. The relay agent can support multiple clients on
different VPNs, and many of these clients from different VPNs can share the same IP address.
Configuring VPNs involves an adjustment to the usual DHCP host IP address designation. VPNs use
private address spaces that might not be unique across the Internet.
In some environments, a relay agent resides in a network element that also has access to one or more MPLS
VPNs. A DHCP server that provides service to DHCP clients on those different VPNs must locate the VPN
in which each client resides. The network element that contains the relay agent typically captures the VPN
association of the DHCP client and includes this information in the relay agent information option of the
DHCP packet.
DHCP relay support for MPLS VPNs allows the relay agent to forward this necessary VPN-related
information to the DHCP server using the following three suboptions of the DHCP relay agent information
option:
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VPN identifier
Subnet selection
Server identifier override
The VPN identifier suboption is used by the relay agent to inform the DHCP server about the VPN for
every DHCP request that the relay agent passes on to the DHCP server; the VPN identifier suboption is
also used to properly forward any DHCP reply that the DHCP server sends back to the relay agent. The
VPN identifier suboption contains the VPN ID configured on the incoming interface to which the client is
connected. If you configure the VPN routing and forwarding (VRF) name but not the VPN ID, the VRF
name is used as the VPN identifier suboption. If the interface is in the global routing space, VPN
suboptions are not added.
The subnet selection suboption allows the separation of the subnet, where the client resides, from the IP
address used to communicate with the relay agent. In typical DHCP processing, the gateway address
specifies both the subnet on which a DHCP client resides and the IP address that the server can use to
communicate with the relay agent. Situations exist where the relay agent needs to specify the subnet on
15
Configuring DHCP Relay Agent Support for MPLS VPNs
How to Configure the DHCP Relay Agent
which a DHCP client resides that is different from the IP address that the server can use to communicate
with the relay agent. The subnet selection suboption is included in the relay agent information option and
passed on to the DHCP server. The gateway address is changed to the outgoing interface of the relay agent
toward the DHCP server. The DHCP server uses this gateway address to send reply packets back to the
relay agent.
The server identifier override suboption value is copied in the reply packet from the DHCP server instead
of the normal server ID address. The server identifier override suboption contains the incoming interface IP
address, which is the IP address on the relay agent that is accessible from the client. Using this information,
the DHCP client sends all renew and release packets to the relay agent. The relay agent adds all the VPN
suboptions to the packets and forwards the packets to the original DHCP server.
After adding these suboptions to the DHCP relay agent information option, the gateway address is changed
to the outgoing interface of the relay agent toward the DHCP server. When the packets are returned from
the DHCP server, the relay agent removes the relay agent information options from the packets and
forwards the packets to the DHCP client on the correct VPN.
The figure below shows a VPN scenario where the DHCP relay agent and DHCP server can recognize the
VPN within which each client resides. DHCP client 1 is part of VPN green, and DHCP client 2 is part of
VPN red, and both have the same private IP address 192.168.1.0/24. Because the clients have the same IP
address, the DHCP relay agent and DHCP server use the VPN identifier, subnet selection, and server
identifier override suboptions of the relay agent information option to distinguish the correct VPN of the
client.
Figure 3
VPN DHCP Configuration
VPN blue/192.168.1.0/24
DHCP client 1
in "green"
VPN red/192.168.1.0/24
DHCP client 2
in "red"
172.27.180.232
172.27.181.73
DHCP relay agent
on router
121983
192.168.1.1
DHCP server
Before configuring DHCP relay support for Multiprotocol Label Switching (MPLS) VPNs, you must
configure standard MPLS VPNs.
16
Configuring DHCP Relay Agent Support for MPLS VPNs
How to Configure the DHCP Relay Agent
Note
•
•
•
If the ip dhcp relay information option vpn global configuration command is configured and the ip
dhcp relay information option vpn-id interface configuration command is not configured, the global
configuration is applied to all interfaces.
If the ip dhcp relay information option vpn global configuration command is configured and the ip
dhcp relay information option vpn-id interface configuration command is also configured, the
interface configuration command takes precedence over the global configuration command. However,
the global configuration is applied to interfaces without the interface configuration.
If the ip dhcp relay information option vpn global configuration command is not configured and the
ip dhcp relay information option vpn-id interface configuration command is configured, only the
interface on which the configuration option is applied is affected. All other interfaces are not impacted
by the configuration.
SUMMARY STEPS
1. enable
2. configure terminal
3. ip dhcp relay information option vpn
4. interface type number
5. ip helper-address vrf name [global] address
6. ip dhcp relay information option vpn-id [none]
7. end
DETAILED STEPS
Command or Action
Step 1 enable
Purpose
Enables privileged EXEC mode.
•
Enter your password if prompted.
Example:
Device> enable
Step 2 configure terminal
Enters global configuration mode.
Example:
Device# configure terminal
Step 3 ip dhcp relay information option vpn
Example:
Device(config)# ip dhcp relay
information option vpn
Enables the system to insert VPN suboptions into the DHCP relay agent
information option in forwarded BOOTREQUEST messages to a DHCP server
and sets the gateway address to the outgoing interface toward the DHCP server.
•
The VPN suboptions are also added to the BOOTP broadcast packets
when the command is configured.
17
Configuring Support for Relay Agent Information Option Encapsulation
How to Configure the DHCP Relay Agent
Command or Action
Step 4 interface type number
Purpose
Configures an interface and enters interface configuration mode.
Example:
Device(config)# interface
FastEthernet0/0
Step 5 ip helper-address vrf name [global]
address
Forwards UDP broadcasts, including BOOTP, received on an interface.
•
Example:
If the DHCP server resides in a different VRF or global space that is
different from the VPN, the vrf name or global options allow you to
specify the name of the VRF or the global space in which the DHCP
server resides.
Device(config-if)# ip helperaddress vrf vrf1 172.27.180.232
Step 6 ip dhcp relay information option vpn- (Optional) Enables the system to insert VPN suboptions into the DHCP relay
agent information option in forwarded BOOTREQUEST messages to a DHCP
id [none]
server and sets the gateway address to the outgoing interface toward the DHCP
server.
Example:
•
Device(config-if)# ip dhcp relay
information option vpn-id
•
•
Step 7 end
The VPN suboptions are also added to the BOOTP broadcast packets
when the command is configured.
The ip dhcp relay information option vpn-id none command allows you
to disable the VPN functionality on the interface. The only time you need
to use this command is when the ip dhcp relay information option vpn
global configuration command is configured and you want to override the
global configuration.
The no ip dhcp relay information option vpn-id command removes the
configuration from the running configuration. In this case, the interface
inherits the global configuration, which may or may not be configured to
insert VPN suboptions.
Returns to privileged EXEC mode.
Example:
Device(config-if)# end
Configuring Support for Relay Agent Information Option Encapsulation
When two relay agents are relaying messages between the DHCP client and the DHCP server, the relay
agent closer to the server, by default, replaces the first option 82 information with its own option 82. The
remote ID and circuit ID information from the first relay agent is lost. In some deployment scenarios, it is
necessary to maintain the initial option 82 from the first relay agent, in addition to the option 82 from the
second relay agent, for example, in a situation where an Intelligent Services Gateway (ISG) acting as a
second relay agent is connected to a Layer 2 device. The Layer 2 device connects to the household and
identifies the household with its own option 82.
18
Configuring Support for Relay Agent Information Option Encapsulation
How to Configure the DHCP Relay Agent
The DHCP Relay Option 82 Encapsulation feature allows the second relay agent to encapsulate option 82
information in a received message from the first relay agent if the second relay agent is configured to add
its own option 82 information. This configuration allows the DHCP server to use option 82 information
from both relay agents. The DHCP server can use the VPN information from the second relay agent, along
with the option 82 information from the first relay agent, to send correct address assignments and other
configuration parameters for the client devices based on the VRF, option 60, and encapsulated option 82.
The reply message from the DHCP server to the DHCP client traverses the same path as the request
messages through the two relay agents to the DHCP client.
The figure below shows the processing that occurs on the two relay agents and the DHCP server when this
feature is configured:
1 The DHCP client generates a DHCP message (including option 60) and broadcasts it on the network.
2 The first DHCP relay agent intercepts the broadcast DHCP request packet and inserts its own option 82
in the packet.
3 The relay agent automatically adds the circuit ID suboption and the remote ID suboption to option 82
and forwards them to the second relay agent.
4 The second relay agent encapsulates the first relay agent’s option 82 and inserts its own option 82.
5 The gateway IP address (giaddr) is set to the incoming interface on the second relay agent and the
original giaddr from the first relay agent is encapsulated.
6 The second DHCP relay agent unicasts the DHCP packet to the DHCP server.
7 The DHCP server receives the packet and uses the VPN suboption information from the second relay
agent, along with the option 82 information from the first relay agent, to assign IP addresses and other
configuration parameters and forwards the packet back to the second relay agent.
8 When the second relay agent receives the reply message from the server, it restores the encapsulated
option 82 and prior giaddr from the first relay agent. The reply message is then sent to the prior giaddr.
9 The first relay agent strips option 82 off from the packet before forwarding the packet to the client.
Figure 4
Processing DHCP Relay Agent Information Option Encapsulation Support
2, 3
4, 5, 6
DHCP
client
1
9
Second DHCP
relay agent
8
DHCP
server
7
204909
First DHCP
relay agent
DHCP
client
19
Configuring Support for Relay Agent Information Option Encapsulation
How to Configure the DHCP Relay Agent
SUMMARY STEPS
1. enable
2. configure terminal
3. ip dhcp relay information option
4. ip dhcp relay information option vpn
5. ip dhcp relay information policy encapsulate
6. interface type number
7. ip dhcp relay information policy-action encapsulate
8. end
DETAILED STEPS
Command or Action
Step 1 enable
Purpose
Enables privileged EXEC mode.
•
Enter your password if prompted.
Example:
Device> enable
Step 2 configure terminal
Enters global configuration mode.
Example:
Device# configure terminal
Step 3 ip dhcp relay information option
Example:
Enables the system to insert the DHCP relay agent information option
(option-82 field) in forwarded BOOTREQUEST messages to a DHCP server.
•
This function is disabled by default.
Device(config)# ip dhcp relay
information option
Step 4 ip dhcp relay information option vpn
Example:
Device(config)# ip dhcp relay
information option vpn
Step 5 ip dhcp relay information policy
encapsulate
Example:
Device(config)# ip dhcp relay
information policy encapsulate
20
(Optional) Enables the system to insert VPN suboptions into the DHCP relay
agent information option in forwarded BOOTREQUEST messages to a DHCP
server and sets the gateway address to the outgoing interface toward the DHCP
server.
•
The VPN suboptions are also added to the BOOTP broadcast packets
when the command is configured.
Enables the system to encapsulate the DHCP relay agent information option
(option-82 field) received from a prior relay agent in forwarded
BOOTREQUEST messages to a DHCP server.
•
Option 82 information from both relay agents will be forwarded to the
DHCP server.
Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay Agent Forwarding
How to Configure the DHCP Relay Agent
Command or Action
Step 6 interface type number
Purpose
(Optional) Configures an interface and enters interface configuration mode.
•
Example:
If you configure the global configuration command, there is no need to
configure the interface configuration command unless you want to apply a
different configuration on a specific interface.
Device(config)# interface
FastEthernet0/0
Step 7 ip dhcp relay information policy-action (Optional) Enables the system to encapsulate the DHCP relay agent
information option (option-82 field) received on an interface from a prior relay
encapsulate
agent in forwarded BOOTREQUEST messages to a DHCP server on an
interface.
Example:
•
Device(config-if)# ip dhcp relay
information policy-action
encapsulate
Step 8 end
This function is disabled by default. This command has precedence over
the global configuration command. However, if the relay agent
information option encapsulation support is configured in global
configuration mode, but not in interface configuration mode, the interface
inherits the global configuration.
Returns to privileged EXEC mode.
Example:
Device(config-if)# end
Setting the Gateway Address of the DHCP Broadcast to a Secondary
Address Using Smart Relay Agent Forwarding
You only need to configure helper addresses on the interface where the UDP broadcasts that you want to
forward to the DHCP server are being received. You only need to configure the ip dhcp smart-relay
command if you have secondary addresses on that interface and you want the router to step through each IP
network when forwarding DHCP requests. If smart relay agent forwarding is not configured, all requests
are forwarded using the primary IP address on the interface.
If the ip dhcp smart-relay command is configured, the relay agent counts the number of times that the
client retries sending a request to the DHCP server when there is no DHCPOFFER message from the
DHCP server. After three retries, the relay agent sets the gateway address to the secondary address. If the
DHCP server still does not respond after three more retries, then the next secondary address is used as the
gateway address.
This functionality is useful when the DHCP server cannot be configured to use secondary pools.
SUMMARY STEPS
1. enable
2. configure terminal
3. ip dhcp smart-relay
4. exit
21
Configuring Support for Private and Standard Suboption Numbers
How to Configure the DHCP Relay Agent
DETAILED STEPS
Command or Action
Step 1 enable
Purpose
Enables privileged EXEC mode.
•
Enter your password if prompted.
Example:
Device> enable
Step 2 configure terminal
Enters global configuration mode.
Example:
Device# configure terminal
Step 3 ip dhcp smart-relay
Allows the DHCP relay agent to switch the gateway address (giaddr field of a
DHCP packet) to a secondary address when there is no DHCPOFFER message
from a DHCP server.
Example:
Device(config)# ip dhcp smart-relay
Step 4 exit
Returns to privileged EXEC mode.
Example:
Device(config)# exit
Configuring Support for Private and Standard Suboption Numbers
Some features that are not standardized will use the private Cisco relay agent suboption numbers. After the
features are standardized, the relay agent suboptions are assigned the Internet Assigned Numbers Authority
(IANA) numbers. Cisco software supports both private and IANA numbers for these suboptions.
Perform this task to configure the DHCP client to use private or IANA standard relay agent suboption
numbers.
SUMMARY STEPS
1. enable
2. configure terminal
3. ip dhcp compatibility suboption link-selection {cisco | standard}
4. exit
22
Troubleshooting the DHCP Relay Agent
How to Configure the DHCP Relay Agent
DETAILED STEPS
Command or Action
Step 1 enable
Purpose
Enables privileged EXEC mode.
•
Enter your password if prompted.
Example:
Device> enable
Step 2 configure terminal
Enters global configuration mode.
Example:
Device# configure terminal
Step 3 ip dhcp compatibility suboption link-selection {cisco |
standard}
Configures the DHCP client to use private or IANA
standard relay agent suboption numbers.
Example:
Device(config)# ip dhcp compatibility suboption linkselection standard
Step 4 exit
(Optional) Exits global configuration mode and returns
to privileged EXEC mode.
Example:
Device(config)# exit
Troubleshooting the DHCP Relay Agent
The show ip route dhcp command is useful to help troubleshoot issues with the DHCP relay agent that
adds routes to clients from unnumbered interfaces. This command displays all routes added to the routing
table by the DHCP server and the relay agent.
SUMMARY STEPS
1. enable
2. show ip route dhcp
3. show ip route dhcp ip-address
4. show ip route vrf vrf-name dhcp
5. clear ip route [vrf vrf-name] dhcp [ip-address]
23
Example: Configuring Support for the Relay Agent Information Option
Configuration Examples for the Cisco IOS DHCP Relay Agent
DETAILED STEPS
Command or Action
Step 1 enable
Purpose
Enables privileged EXEC mode.
•
Enter your password if prompted.
Example:
Device> enable
Step 2 show ip route dhcp
Displays all routes added by the Cisco IOS DHCP server and relay
agent.
Example:
Device# show ip route dhcp
Step 3 show ip route dhcp ip-address
Displays all routes added by the Cisco IOS DHCP server and relay agent
associated with the specified IP address.
Example:
Device# show ip route dhcp 172.16.1.3
Step 4 show ip route vrf vrf-name dhcp
Displays all routes added by the Cisco IOS DHCP server and relay agent
associated with the named VRF.
Example:
Device# show ip route vrf vrf1 dhcp
Step 5 clear ip route [vrf vrf-name] dhcp [ip-address] Removes routes from the routing table added by the DHCP server and
relay agent for DHCP clients on unnumbered interfaces.
Example:
Device# clear ip route dhcp
Configuration Examples for the Cisco IOS DHCP Relay Agent
Example: Configuring Support for the Relay Agent Information Option
The following example shows how to enable the DHCP server, the relay agent, and the insertion and
removal of the DHCP relay information option (option 82). Note that the Cisco IOS DHCP server is
enabled by default. In this example, the DHCP server is disabled:
! Reenables the DHCP server.
service dhcp
ip dhcp relay information option
!
interface ethernet0/0
24
Example: Configuring Per-Interface Support for the Relay Agent Information Option
Configuration Examples for the Cisco IOS DHCP Relay Agent
ip address 192.168.100.1 255.255.255.0
ip helper-address 10.55.11.3
Example: Configuring Per-Interface Support for the Relay Agent Information
Option
The following example shows that for subscribers who are being serviced by the same aggregation router,
the relay agent information option for ATM subscribers must be processed differently from that for
Ethernet digital subscribers. For ATM subscribers, the relay agent information option is configured to be
removed from the packet by the relay agent before forwarding the packet to the client. For Ethernet
subscribers, the connected device provides the relay agent information option, and the option is configured
to remain in the packet and be forwarded to the client.
ip dhcp relay information trust-all
interface Loopback0
ip address 10.16.0.1 255.255.255.0
!
interface ATM3/0
no ip address
!
interface ATM3/0.1
ip helper-address 10.16.1.2
ip unnumbered loopback0
ip dhcp relay information option-insert
!
interface Loopback1
ip address 10.18.0.1 255.255.255.0
!
interface Ethernet4
no ip address
!
interface Ethernet4/0.1
encapsulation dot1q 123
ip unnumbered loopback1
ip helper-address 10.18.1.2
ip dhcp relay information policy-action keep
Example: Configuring the Subscriber Identifier Suboption of the Relay Agent
Information Option
The following example shows how to add a unique identifier to the subscriber-identifier suboption of the
relay agent information option:
ip dhcp relay information option
!
interface Loopback0
ip address 10.1.1.129 255.255.255.192
!
interface ATM4/0
no ip address
!
interface ATM4/0.1 point-to-point
ip helper-address 10.16.1.2
ip unnumbered Loopback0
ip dhcp relay information option subscriber-id newperson123
atm route-bridged ip
pvc 88/800
encapsulation aal5snap
25
Example: Configuring DHCP Relay Class Support for Client Identification
Configuration Examples for the Cisco IOS DHCP Relay Agent
Example: Configuring DHCP Relay Class Support for Client Identification
In the following example, DHCP messages are received from DHCP clients on subnet 10.2.2.0. The relay
agent will match and identify the relay class from the relay pool and forward the DHCP message to the
appropriate DHCP server identified by the relay target command.
!
ip dhcp class H323
option 60 hex 010203
!
ip dhcp class SIP
option 60 hex 040506
!
! The following is the relay pool:
ip dhcp pool pool1
relay source 10.2.2.0 255.255.255.0
class H323
relay target 192.168.2.1
relay target 192.168.3.1
!
class SIP
relay target 192.168.4.1
Example: Configuring DHCP Relay Agent Support for MPLS VPNs
In the following example, the DHCP relay agent receives a DHCP request on Ethernet interface 0/1 and
sends the request to the DHCP server located at IP helper address 10.44.23.7, which is associated with the
VRF named vrf1:
ip dhcp relay information option vpn
!
interface ethernet 0/1
ip helper-address vrf vrf1 10.44.23.7
!
Example: Configuring Support for Relay Agent Information Option
Encapsulation
In the following example, DHCP relay agent 1 is configured globally to insert the relay agent information
option into the DHCP packet. DHCP relay agent 2 is configured to add its own relay agent information
option, including the VPN information, and to encapsulate the relay agent information option received from
DHCP relay agent 1. The DHCP server receives the relay agent information options from both the relay
agents, uses this information to assign IP addresses and other configuration parameters, and forwards them
back to the client.
DHCP Relay Agent 1
ip dhcp relay information option
DHCP Relay Agent 2
ip dhcp relay information option
ip dhcp relay information option vpn
ip dhcp relay information option encapsulation
26
Example: Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay Agent
Forwarding
Additional References
Example: Setting the Gateway Address of the DHCP Broadcast to a
Secondary Address Using Smart Relay Agent Forwarding
In the following example, the router will forward the DHCP broadcast received on Ethernet interface 0/0 to
the DHCP server (10.55.11.3), by inserting 192.168.100.1 in the giaddr field of the DHCP packet. If the
DHCP server has a scope or pool configured for the 192.168.100.0/24 network, the server will respond;
otherwise, it will not respond.
Because the ip dhcp smart-relay global configuration command is configured, if the router sends three
requests using 192.168.100.1 in the giaddr field and does not get a response, the router will move on and
start using 172.16.31.254 in the giaddr field instead. Without the smart relay functionality, the router uses
only 192.168.100.1 in the giaddr field.
ip dhcp smart-relay
!
interface ethernet0/0
ip address 192.168.100.1 255.255.255.0
ip address 172.16.31.254 255.255.255.0
ip helper-address 10.55.11.3
!
Additional References
Related Documents
Related Topic
Document Title
Cisco IOS commands
Cisco IOS Master Commands List, All Releases
DHCP commands: complete command syntax,
command modes, command history, defaults, usage
guidelines, and examples
DHCP conceptual information
“DHCP Overview” module in the Cisco IOS IP
Addressing Configuration Guide
DHCP server configuration
“Configuring the Cisco IOS DHCP Server” module
in the Cisco IOS IP Addressing Configuration
Guide
DHCP client configuration
“Configuring the Cisco IOS DHCP Client” module
in the Cisco IOS IP Addressing Configuration
Guide
DHCP server on-demand address pool manager
configuration
“Configuring the DHCP Server On-Demand
Address Pool Manager” module in the Cisco IOS IP
Addressing Configuration Guide
DHCP advanced features
“Configuring DHCP Services for Accounting and
Security” module in the Cisco IOS IP Addressing
Configuration Guide
27
Example: Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay Agent
Forwarding
Technical Assistance
Related Topic
Document Title
DHCP enhancements for edge-session management “Configuring DHCP Enhancements for Edgeconfiguration
Session Management” module in the Cisco IOS IP
Addressing Configuration Guide
DHCP options
" DHCP Options” appendix in the Network
Registrar User’s Guide, Release 6.1.1
DHCP for IPv6
“Implementing DHCP for IPv6” module in the
Cisco IOS IPv6 Configuration Guide
Standards
Standards
Title
No new or modified standards are supported by this —
functionality.
MIBs
MIBs
MIBs Link
No new or modified MIBs are supported by this
feature.
To locate and download MIBs for selected
platforms, Cisco IOS releases, and feature sets, use
Cisco MIB Locator found at the following URL:
http://www.cisco.com/go/mibs
RFCs
RFCs
Title
RFC 951
Bootstrap Protocol (BOOTP)
RFC 1542
Clarifications and Extensions for the Bootstrap
Protocol
RFC 2131
Dynamic Host Configuration Protocol
RFC 2685
Virtual Private Networks Identifier
RFC 3046
DHCP Relay Information Option
RFC 5460
DHCPv6 Bulk Leasequery
Technical Assistance
28
Example: Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay Agent
Forwarding
Feature Information for the Cisco IOS DHCP Relay Agent
Description
Link
The Cisco Support and Documentation website
provides online resources to download
documentation, software, and tools. Use these
resources to install and configure the software and
to troubleshoot and resolve technical issues with
Cisco products and technologies. Access to most
tools on the Cisco Support and Documentation
website requires a Cisco.com user ID and
password.
http://www.cisco.com/cisco/web/support/
index.html
Feature Information for the Cisco IOS DHCP Relay Agent
The following table provides release information about the feature or features described in this module.
This table lists only the software release that introduced support for a given feature in a given software
release train. Unless noted otherwise, subsequent releases of that software release train also support that
feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support.
To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1
Feature Information for the Cisco IOS DHCP Relay Agent
Feature Name
Releases
Feature
Information
DHCP Relay
Option 82
Encapsulation
12.2(33)SRD
This feature
allows a second
DHCP relay
agent to
encapsulate the
relay agent
information
option (option
82) from a prior
relay agent, add
its own option
82, and forward
the packet to
the DHCP
server. The
DHCP server
can use the
VPN
information
from the second
relay agent
along with the
option 82
29
Example: Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay Agent
Forwarding
Feature Information for the Cisco IOS DHCP Relay Agent
Feature Name
Releases
Feature
Information
information
from the first
relay agent to
send correct
address
assignments
and other
configuration
parameters for
the client
devices based
on the VRF,
option 60, and
encapsulated
option 82.
The following
commands
were modified
by this
feature:ip dhcp
relay
information
policy, ip dhcp
relay
information
policy-action.
30
DHCP Class
Support for
Client
Identification
12.4(11)T
This feature
enhances the
DHCP class
mechanism to
support options
60, 77, 124, and
125. These
options identify
the type of
client sending
the DHCP
message. The
DHCP relay
agent can make
forwarding
decisions based
on the content
of the options
in the DHCP
message sent by
the client.
The following
command was
introduced by
this feature:
option hex.
Example: Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay Agent
Forwarding
Feature Information for the Cisco IOS DHCP Relay Agent
Feature Name
Releases
DHCPv4 Relay 12.4(11)T
per Interface
VPN ID
Support
Feature
Information
The DHCPv4
Relay per
Interface VPN
ID Support
feature allows
the Cisco IOS
DHCP relay
agent to be
configured per
interface to
override the
global
configuration of
the ip dhcp
relay
information
option vpn
command. This
feature allows
subscribers
with different
relay
information
option VPN ID
requirements on
different
interfaces to be
reached from
one Cisco
router.
The following
command was
introduced by
this feature: ip
dhcp relay
information
option vpn-id.
31
Example: Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay Agent
Forwarding
Feature Information for the Cisco IOS DHCP Relay Agent
Feature Name
Releases
Feature
Information
DHCP Relay
Option 82 per
Interface
Support
12.4(6)T
12.2(31)SB2
12.2(33)SRC
This feature
enables support
for the DHCP
relay agent
information
option (option
82) on a per
interface basis.
The interface
configuration
allows different
DHCP servers,
with different
DHCP option
82 requirements
to be reached
from one Cisco
router.
The following
commands
were introduced
by this feature:
ip dhcp relay
information
check-reply, ip
dhcp relay
information
option-insert,
ip dhcp relay
information
policy-action.
32
Example: Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay Agent
Forwarding
Feature Information for the Cisco IOS DHCP Relay Agent
Feature Name
Releases
Feature
Information
DHCP
Subscriber
Identifier
Suboption of
Option 82
12.3(14)T
12.2(28)SB
12.2(33)SRB
This feature
enables an ISP
to add a unique
identifier to the
subscriberidentifier
suboption of the
relay agent
information
option.
The following
command was
introduced by
this feature: ip
dhcp relay
information
option
subscriber-id.
33
Example: Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay Agent
Forwarding
Feature Information for the Cisco IOS DHCP Relay Agent
Feature Name
Releases
Feature
Information
DHCP Relay
MPLS VPN
Support
12.2(8)
12.2(28)SB
12.2(33)SRC
DHCP relay
support for
MPLS VPNs
enables a
network
administrator to
conserve
address space
by allowing
overlapping
addresses. The
relay agent can
support
multiple clients
on different
VPNs, and
many of these
clients from
different VPNs
can share the
same IP
address.
The following
commands
were modified
by this feature:
ip dhcp relay
information
option, ip
helper address.
34
Example: Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay Agent
Forwarding
Glossary
Feature Name
Releases
Feature
Information
DHCPv6 Bulk
Lease query
15.1(1)S
Cisco IOS
DHCPv6 relay
agent supports
bulk lease
query in
accordance
with RFC 5460.
The following
commands
were introduced
or modified by
this feature:
debug ipv6
dhcp relay ,
ipv6 dhcprelay bulklease.
Glossary
client—A host that is trying to configure its interface (obtain an IP address) using DHCP or BOOTP
protocols.
DHCP—Dynamic Host Configuration Protocol. A network protocol that automatically provides an IP host
with an IP address and other related configuration information (for example, subnet mask and default
gateway).
giaddr—gateway IP address. The giaddr field of the DHCP message provides the DHCP server with
information about the IP address subnet on which the client is to reside. It also provides the DHCP server
with an IP address where the response messages are to be sent.
MPLS—Multiprotocol Label Switching. Industry standard upon which tag switching is based.
relay agent—A router that forwards DHCP and BOOTP messages between a server and a client on
different subnets.
server—A DHCP or BOOTP server.
VPN—Virtual Private Network. Enables IP traffic to use tunneling to travel securely over a public TCP/IP
network.
VRF—VPN routing and forwarding instance. A VRF consists of an IP routing table, a derived forwarding
table, a set of interfaces that use the forwarding table, and a set of rules and routing protocols that
determine what goes into the forwarding table. In general, a VRF includes the routing information that
defines a customer VPN site that is attached to a Provider Edge (PE) router. Each VPN that is instantiated
on the PE router has its own VRF.
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Example: Setting the Gateway Address of the DHCP Broadcast to a Secondary Address Using Smart Relay Agent
Forwarding
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Third-party trademarks mentioned are the property of their respective owners. The use of the word partner
does not imply a partnership relationship between Cisco and any other company. (1110R)
Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be
actual addresses and phone numbers. Any examples, command display output, network topology diagrams,
and other figures included in the document are shown for illustrative purposes only. Any use of actual IP
addresses or phone numbers in illustrative content is unintentional and coincidental.
© 2012 Cisco Systems, Inc. All rights reserved.
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