C H A P T E R
7
Configuring Spanning Tree
This chapter describes the IEEE 802.1D bridge Spanning Tree Protocol (STP) and how to use and
configure Cisco’s proprietary STPs, Per VLAN Spanning Tree + (PVST+), and Multi-Instance Spanning
Tree Protocol (MISTP) on the Catalyst enterprise LAN switches.
Note
For information on configuring the spanning tree PortFast, UplinkFast, and BackboneFast features, see
Chapter 8, “Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast,
BackboneFast, and Loop Guard.”
This chapter consists of these sections:
Note
•
Understanding How STPs Work, page 7-2
•
Understanding How PVST+ and MISTP Modes Work, page 7-11
•
Understanding How Bridge Identifiers Work, page 7-13
•
Understanding How MST Works, page 7-14
•
Using MISTP-PVST+ or MISTP, page 7-30
•
Configuring a Root Switch, page 7-39
•
Configuring Spanning Tree Timers, page 7-44
•
Understanding How BPDU Skewing Works, page 7-22
•
Configuring Spanning Tree BPDU Skewing, page 7-57
•
Configuring MST, page 7-46
For complete syntax and usage information for the commands that are used in this chapter, refer to the
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Command
Reference.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-1
Chapter 7
Configuring Spanning Tree
Understanding How STPs Work
Understanding How STPs Work
This section describes the specific functions that are common to all spanning tree protocols. The Cisco
proprietary spanning tree protocols, PVST+ and MISTP, are based on the IEEE 802.1D STP. (See the
“Understanding How PVST+ and MISTP Modes Work” section on page 7-11 for information about
PVST+ and MISTP.) The 802.1D STP is a Layer 2 management protocol that provides path redundancy
in a network while preventing undesirable loops. All spanning tree protocols use an algorithm that
calculates the best loop-free path through the network.
STP uses a distributed algorithm that selects one bridge of a redundantly connected network as the root
of a spanning tree-connected active topology. STP assigns roles to each port depending on what the
port’s function is in the active topology. Port roles are as follows:
•
Root—A forwarding port that is elected for the spanning tree topology
•
Designated—A forwarding port that is elected for every switched LAN segment
•
Alternate—A blocked port providing an alternate path to the root port in the spanning tree
•
Backup—A blocked port in a loopback configuration
Switches that have ports with these assigned roles are called root or designated switches. For more
information, see the “Understanding How a Topology Is Created” section on page 7-2.
In Ethernet networks, only one active path may exist between any two stations. Multiple active paths
between stations can cause loops in the network. When loops occur, some switches recognize stations
on both sides of the switch. This situation causes the forwarding algorithm to malfunction allowing
duplicate frames to be forwarded.
Spanning tree algorithms provide path redundancy by defining a tree that spans all of the switches in an
extended network and then forces certain redundant data paths into a standby (blocked) state. At regular
intervals, the switches in the network send and receive spanning tree packets which they use to identify
the active path. If one network segment becomes unreachable, or if spanning tree costs change, the
spanning tree algorithm reconfigures the spanning tree topology and reestablishes the link by activating
a standby path.
Spanning tree operation is transparent to end stations, which do not detect whether they are connected
to a single LAN segment or a switched LAN of multiple segments.
Understanding How a Topology Is Created
All switches in an extended LAN participating in a spanning tree gather information about other
switches in the network through an exchange of data messages known as bridge protocol data units
(BPDUs). This exchange of messages results in the following actions:
7-2
•
A unique root switch is elected for the spanning tree network topology.
•
A designated switch is elected for every switched LAN segment.
•
Any loops in the switched network are eliminated by placing redundant switch ports in a backup
state; all paths that are not needed to reach the root switch from anywhere in the switched network
are placed in STP-blocked mode.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Understanding How STPs Work
The following three things determine the topology of an active switched network:
•
The unique switch identifier (MAC address of the switch) that is associated with each switch
•
The path cost to the root that is associated with each switch port
•
The port identifier (MAC address of the port) that is associated with each switch port
In a switched network, the root switch is the logical center of the spanning tree topology. A spanning
tree protocol uses BPDUs to elect the root switch and root port for the switched network and the root
port and designated port for each switched segment.
Understanding How a Switch or Port Becomes the Root Switch or Root Port
If all switches in a network are enabled with default settings, the switch with the lowest MAC address
becomes the root switch. In Figure 7-1, Switch A, with the lowest MAC address, is the root switch.
However, due to traffic patterns, number of forwarding ports, or line types, Switch A might not be the
ideal root switch. You can force a switch to become the root switch by increasing the priority (lowering
the priority number) on the preferred switch. This action causes the spanning tree to recalculate the
topology and make the selected switch the root switch.
Figure 7-1
Configuring a Loop-Free Topology
DP
A
DP
RP
DP
RP
B
D
DP DP
DP
RP
C
S5688
DP
RP = Root Port
DP = Designated Port
You can also change the priority of a port to make it the root port. When the spanning tree topology is
based on default parameters, the path between the source and the destination stations in a switched
network might not be ideal. The goal is to make the fastest link the root port, connecting higher-speed
links to a port that has a higher number than the current root port can cause a root-port change.
For example, assume that a port on Switch B is a fiber-optic link. Also, another port on Switch B (an
unshielded twisted-pair [UTP] link) is the root port. Network traffic might be more efficient over the
high-speed fiber-optic link. By changing the Port Priority parameter for the UTP port to a higher priority
(lower numerical value) than the fiber-optic port, the UTP port becomes the root port. You could also
accomplish this scenario by changing the port cost parameter for the UTP port to a lower value than that
of the fiber-optic port.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-3
Chapter 7
Configuring Spanning Tree
Understanding How STPs Work
Understanding BPDUs
BPDUs contain configuration information about the transmitting switch and its ports, including switch
and port MAC addresses, switch priority, port priority, and port cost. Each configuration BPDU contains
this information:
•
The unique identifier of the switch that the transmitting switch believes to be the root switch
•
The cost of the path to the root from the transmitting port
•
The identifier of the transmitting port
The switch sends configuration BPDUs to communicate with and compute the spanning tree topology.
A MAC frame conveying a BPDU sends the switch group address to the destination address field. All
switches connected to the LAN on which the frame is transmitted receive the BPDU. BPDUs are not
directly forwarded by the switch, but the receiving switch uses the information in the frame to calculate
a BPDU. If the topology changes, the receiving switch initiates a BPDU transmission.
A BPDU exchange results in the following:
•
One switch is elected as the root switch.
•
The shortest distance to the root switch is calculated for each switch.
•
A designated switch is selected. This is the switch that is closest to the root switch through which
frames will be forwarded to the root.
•
A port for each switch is selected. This is the port that provides the best path from the switch to the
root switch.
•
Ports included in the STP are selected.
Calculating and Assigning Port Costs
By calculating and assigning the port cost of the switch ports, you can ensure that the shortest (lowest
cost) distance to the root switch is used to transmit data. You can calculate and assign lower path cost
values (port costs) to higher bandwidth ports by using either the short method (which is the default) or
the long method. The short method uses a 16-bit format that yields values from 1–65535. The long
method uses a 32-bit format that yields values from 1–200,000,000. For more information on setting the
default cost mode, see the “Configuring the PVST+ Default Port Cost Mode” section on page 7-26.
Note
You should configure all switches in your network to use the same method for calculating port cost. The
short method (default) will be used to calculate the port cost unless you specify the long method. You
can specify the calculation method using the CLI.
Calculating the Port Cost Using the Short Method
The IEEE 802.1D specification assigns 16-bit (short) default port cost values to each port that is based
on bandwidth. You can also manually assign port costs between 1–65535. The 16-bit values are only
used for ports that have not been specifically configured for port cost. Table 7-1 shows the default port
cost values that are assigned by the switch for each type of port when you use the short method to
calculate the port cost.
7-4
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Understanding How STPs Work
Table 7-1
Default Port Cost Values Using the Short Method
Port Speed
Default Cost Value
Default Range
10 Mbps
100
1 to 65535
100 Mbps
19
1 to 65535
1 Gbps
4
1 to 65535
Calculating the Port Cost Using the Long Method
802.1t assigns 32-bit (long) default port cost values to each port using a formula that is based on the port
bandwidth. You can also manually assign port costs between 1–200,000,000. The formula for obtaining
default 32-bit port costs is to divide the bandwidth of the port by 200,000,000. Table 7-2 shows the
default port cost values that are assigned by the switch and the recommended cost values and ranges for
each type of port when you use the long method to calculate port cost.
Table 7-2
Default Port Cost Values Using the Long Method
Port Speed
Recommended Value
Recommended Range
Available Range
≤100 kbps
200000000
20000000 to 200000000
1 to 200000000
1 Mbps
20000000
2000000 to 200000000
1 to 200000000
10 Mbps
2000000
200000 to 20000000
1 to 200000000
100 Mbps
200000
20000 to 2000000
1 to 200000000
1 Gbps
20000
2000 to 200000
1 to 200000000
10 Gbps
2000
200 to 20000
1 to 200000000
Calculating the Port Cost for Aggregate Links
As individual links are added or removed from an aggregate link (port bundle), the bandwidth of the
aggregate link increases or decreases. These changes in bandwidth lead to the recalculation of the default
port cost for the aggregated port. Changes to the default port cost or changes resulting from links that
autonegotiate their bandwidth could lead to recalculation of the spanning tree topology. Recalculation
may not be desirable, especially if the added or removed link is of little consequence to the bandwidth
of the aggregate link (for example, if a 10-Mbps link is removed from a 10-Gbps aggregate link).
Because of the limitations that are presented by automatically recalculating the topology, 802.1t states
that changes in bandwidth will not result in changes to the cost of the port concerned. Therefore, the
aggregated port uses the same port cost parameters as a standalone port.
Understanding Spanning Tree Port States
Topology changes can take place in a switched network due to a link coming up or going down (failing).
When a switch port transitions directly from nonparticipation in the topology to the forwarding state, it
can create temporary data loops. Ports must wait for new topology information to propagate through the
switches in the LAN before they can start forwarding frames. They must also allow the frame lifetime
to expire for frames that have been forwarded using the old topology.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-5
Chapter 7
Configuring Spanning Tree
Understanding How STPs Work
At any given time, each port on a switch using STP is in one of these states:
•
Blocking
•
Listening
•
Learning
•
Forwarding
•
Disabled
A port moves through these states:
•
From initialization to blocking
•
From blocking to either listening or disabled
•
From listening to either listening or disabled
•
From learning to either forwarding or disabled
•
From forwarding to disabled
Figure 7-2 illustrates how a port moves through the states.
Figure 7-2
STP Port States
Boot-up
initialization
Blocking
state
Listening
state
Disabled
state
Forwarding
state
S5691
Learning
state
You can modify each port state by using management software, such as the VLAN Trunking Protocol
(VTP). When you enable spanning tree, every switch in the network goes through the blocking state and
the transitory states of listening and learning at power up. If properly configured, each port stabilizes
into the forwarding or blocking state.
When the spanning tree algorithm places a port in the forwarding state, the following occurs:
7-6
•
The port is put into the listening state while it waits for protocol information that suggests it should
go to the blocking state
•
The port waits for the expiration of a protocol timer that moves the port to the learning state
•
In the learning state, the port continues to block frame forwarding as it learns station location
information for the forwarding database
•
The expiration of a protocol timer moves the port to the forwarding state, where both learning and
forwarding are enabled
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Understanding How STPs Work
Blocking State
A port in the blocking state, such as Port 2 in Figure 7-3, does not participate in frame forwarding. After
initialization, a BPDU is sent to each port in the switch. A switch initially assumes that it is the root until
it exchanges BPDUs with other switches. This exchange establishes which switch in the network is
really the root. If only one switch resides in the network, no exchange occurs, the forward delay timer
expires, and the ports move to the listening state. A switch always enters the blocking state following
switch initialization.
Figure 7-3
Port 2 in Blocking State
Segment
frames
Forwarding
Port 1
Station
addresses
BPDUs
Network
management
and data frames
Filtering
database
System
module
Frame
forwarding
BPDUs
Blocking
S5692
Data
frames
Network
management
frames
Port 2
Segment
frames
A port in the blocking state performs as follows:
•
Discards frames that are received from the attached segment
•
Discards frames that are switched from another port for forwarding
•
Does not incorporate station location into its address database (there is no learning on a blocking
port, so there is no address database update)
•
Receives BPDUs and directs them to the system module
•
Does not transmit BPDUs that are received from the system module
•
Receives and responds to network management messages
Listening State
The listening state is the first transitional state that a port enters after the blocking state. The port enters
this state when the spanning tree determines that the port should participate in frame forwarding.
Learning is disabled in the listening state. Figure 7-4 shows a port in the listening state.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-7
Chapter 7
Configuring Spanning Tree
Understanding How STPs Work
Figure 7-4
Port 2 in Listening State
All segment
frames
Forwarding
Port 1
Station
addresses
BPDUs
Network
management
and data frames
Filtering
database
System
module
Frame
forwarding
BPDUs
Network
management
frames
S5693
Data
frames
Port 2
Listening
All segment
frames
BPDU and network
management frames
A port in the listening state performs as follows:
•
Discards frames that are received from the attached segment
•
Discards frames that are switched from another port for forwarding
•
Does not incorporate station location into its address database (there is no learning at this point, so
there is no address database update)
•
Receives BPDUs and directs them to the system module
•
Processes BPDUs that are received from the system module
•
Receives and responds to network management messages
Learning State
A port in the learning state prepares to participate in frame forwarding. The port enters the learning state
from the listening state. Figure 7-5 shows a port in the learning state.
7-8
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Understanding How STPs Work
Figure 7-5
Port 2 in Learning State
All segment
frames
Forwarding
Port 1
Station
addresses
BPDUs
Network
management
and data frames
Filtering
database
System
module
Frame
forwarding
Station
addresses
BPDUs
Network
management
frames
S5694
Data
frames
Port 2
Learning
All segment
frames
BPDU and network
management frames
A port in the learning state performs as follows:
•
Discards frames that are received from the attached segment
•
Discards frames that are switched from another port for forwarding
•
Incorporates station location into its address database
•
Receives BPDUs and directs them to the system module
•
Receives, processes, and transmits BPDUs that are received from the system module
•
Receives and responds to network management messages
Forwarding State
A port in the forwarding state forwards frames, as shown in Figure 7-6. The port enters the forwarding
state from the learning state.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-9
Chapter 7
Configuring Spanning Tree
Understanding How STPs Work
Figure 7-6
Port 2 in Forwarding State
All segment
frames
Forwarding
Port 1
Station
addresses
BPDUs
Network
management
and data frames
Filtering
database
System
module
Frame
forwarding
Network
management
and data frames
S5695
BPDUs
Station
addresses
Port 2
Forwarding
All segment
frames
A port in the forwarding state performs as follows:
Caution
7-10
•
Forwards frames that are received from the attached segment
•
Forwards frames that are switched from another port for forwarding
•
Incorporates station location information into its address database
•
Receives BPDUs and directs them to the system module
•
Processes BPDUs that are received from the system module
•
Receives and responds to network management messages
Use spanning tree PortFast mode only on ports that are directly connected to individual workstations to
allow these ports to come up and go directly to the forwarding state, instead of going through the entire
spanning tree initialization. To prevent illegal topologies, enable spanning tree on ports that are
connected to switches or other devices that forward messages. For more information on PortFast, see
Chapter 8, “Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast,
BackboneFast, and Loop Guard.”
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Understanding How PVST+ and MISTP Modes Work
Disabled State
A port in the disabled state does not participate in frame forwarding or STP, as shown in Figure 7-7. A
port in the disabled state is virtually nonoperational.
Figure 7-7
Port 2 in Disabled State
All segment
frames
Forwarding
Port 1
Station
addresses
BPDUs
Network
management
and data frames
Filtering
database
System
module
Frame
forwarding
Data
frames
Disabled
S5696
Network
management
frames
Port 2
All segment
frames
A disabled port performs as follows:
•
Discards frames that are received from the attached segment
•
Discards frames that are switched from another port for forwarding
•
Does not incorporate station location into its address database (there is no learning, so there is no
address database update)
•
Receives BPDUs but does not direct them to the system module
•
Does not receive BPDUs for transmission from the system module
•
Receives and responds to network management messages
Understanding How PVST+ and MISTP Modes Work
Catalyst 4500 series switches provide two proprietary spanning tree modes based on the IEEE 802.1D
standard and one mode that is a combination of the two modes:
•
Per VLAN Spanning Tree (PVST+)
•
Rapid PVST+
•
Multi-Instance Spanning Tree Protocol (MISTP)
•
MISTP-PVST+ (combination mode)
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-11
Chapter 7
Configuring Spanning Tree
Understanding How PVST+ and MISTP Modes Work
The following sections provide an overview of each mode.
Caution
If your network currently uses PVST+ and you plan to use MISTP on any switch, you must first enable
MISTP-PVST+ on the switch and configure an MISTP instance to avoid causing network loops.
PVST+ Mode
PVST+ is the default STP used on all Ethernet, Fast Ethernet, and Gigabit Ethernet port-based VLANs
on Catalyst 4500 series switches. PVST+ runs on each VLAN on the switch, ensuring that each has a
loop-free path through the network.
PVST+ provides Layer 2 load balancing for the VLAN on which it runs; you can create different logical
topologies using the VLANs on your network to ensure that all of your links will be used but no one link
will be oversubscribed.
Each instance of PVST+ on a VLAN has a single root switch. This root switch propagates the spanning
tree information that is associated with that VLAN to all other switches in the network. Because each
switch has the same knowledge about the network, this process ensures that the network topology is
maintained.
Rapid PVST+
Rapid PVST+ is the same as PVST+, except that Rapid PVST+ utilizes a Rapid STP that is based on
IEEE 802.1w instead of 802.1D. Rapid PVST+ uses the same configuration as PVST+, and you need
only minimal extra configuration. With Rapid PVST+, dynamic CAM entries are flushed immediately
per port upon any topology change. UplinkFast and BackboneFast are enabled but not active in this
mode, because the functionality is built into the rapid STP. This method provides for quick recovery of
connectivity following the failure of a bridge, bridge port, or LAN.
MISTP Mode
MISTP is an optional STP that runs on Catalyst 4500 series switches. MISTP allows you to group
multiple VLANs under a single instance of spanning tree (an MISTP instance). MISTP combines the
Layer 2 load-balancing benefits of PVST+ with the lower CPU load of IEEE 802.1Q.
An MISTP instance is a virtual logical topology that is defined by a set of bridge and port parameters;
an MISTP instance becomes a real topology when VLANs are mapped to it. Each MISTP instance has
its own root switch and a different set of forwarding links (that is, different bridge and port parameters).
Each MISTP instance has a single root switch, which propagates the information that is associated with
that instance of MISTP to all other switches in the network. This process ensures that the network
topology is maintained because each switch has the same knowledge about the network.
MISTP builds MISTP instances by exchanging MISTP BPDUs with peer entities in the network. There
is only one BPDU for each MISTP instance, rather than for each VLAN as in PVST+. There are fewer
BPDUs in an MISTP network; therefore, there is less overhead in the network. MISTP discards any
PVST+ BPDUs that it sees.
7-12
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Understanding How Bridge Identifiers Work
An MISTP instance can have any number of VLANs that are mapped to it, but a VLAN can only be
mapped to a single MISTP instance. You can easily move a VLAN (or VLANs) in an MISTP topology
to another MISTP instance if it has converged. (However, if ports are added at the same time that the
VLAN is moved, convergence time is required.)
MISTP-PVST+ Mode
MISTP-PVST+ is a transition spanning tree mode that allows you to use the MISTP functionality on
Catalyst 4500 series switches while continuing to communicate with the older Catalyst 5000 family and
6500 series switches in your network that use PVST+. A switch using PVST+ mode and a switch using
MISTP mode connected together cannot see the BPDUs of the other switch, a condition that can cause
loops in the network. MISTP-PVST+ allows interoperability between PVST+ and pure MISTP, because
it detects the BPDUs of both modes. If you wish to convert your network to MISTP, you can use
MISTP-PVST+ to transition the network from PVST+ to MISTP to avoid problems.
MISTP-PVST+ conforms to the limits of PVST+; for example, you can only configure the amount of
VLAN ports on your MISTP-PVST+ switches that you configure on your PVST+ switches.
Understanding How Bridge Identifiers Work
The next two sections explain how MAC addresses are used in PVST+ and MISTP as unique bridge
identifiers.
MAC Address Allocation
Catalyst 4000 series switches have a pool of 1024 MAC addresses that can be used as bridge identifiers
for VLANs running under PVST+ or for MISTP instances. The Catalyst 4500 series switches have a pool
of only 64 MAC addresses. You can use the show module command to view the MAC address range.
MAC addresses are allocated sequentially, with the first MAC address in the range assigned to VLAN 1,
the second in the range assigned to VLAN 2, and so forth. The last MAC address in the range is assigned
to the supervisor engine in-band (sc0) management interface.
For example, if the MAC address range for the supervisor engine is 00-e0-1e-9b-2e-00 to
00-e0-1e-9b-31-ff, the VLAN 1 bridge ID is 00-e0-1e-9b-2e-00, the VLAN 2 bridge ID is
00-e0-1e-9b-2e-01, the VLAN 3 bridge ID is 00-e0-1e-9b-2e-02, and so forth. The in-band (sc0)
interface MAC address is 00-e0-1e-9b-31-ff.
MAC Address Reduction
MAC address reduction is used on Catalyst 6500 series switches to enable extended-range VLAN
identification. If you have a Catalyst 6500 series switch in your network and you have MAC address
reduction enabled on it, you should also enable MAC address reduction on all your Catalyst 4500 series
switches to avoid problems in the spanning tree topology. When MAC address reduction is enabled on
Catalyst 4500 series switches, it disables the pool of MAC addresses that are used for the VLAN
spanning tree, leaving a single MAC address that identifies the switch. For detailed information on MAC
address reduction, refer to the Catalyst 6500 Series Switch Software Configuration Guide.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-13
Chapter 7
Configuring Spanning Tree
Understanding How MST Works
MAC address reduction is always enabled on the Catalyst 4500 series switches; however, it may or may
not be enabled on a Catalyst 4006 switch; this can affect the selection of the root bridge after you migrate
your supervisor engine. Here are two scenarios to consider:
•
The Catalyst 4006 switch is not a root switch
In this case, the spanning tree topology does not change. If you add a Catalyst 4500 series switch
with MAC address reduction enabled and its default spanning tree bridge ID priority set to 32,768
to the network, the bridge ID priority of the new switch becomes the bridge ID priority that is added
to the system ID extension. The system ID extension is the VLAN number and can vary from 1 to
4094. If the switch is in VLAN 1, the new bridge ID priority will be 32,769. Because 32,769 is
greater than 32,768, this switch cannot become the root switch.
•
The Catalyst 4006 is a root switch
In this case, the spanning tree topology might change. If the other switches in the network are not
running MAC address reduction, the topology will change after you replace the chassis with a
Catalyst 4500 series switch. The bridge ID priority of the new Catalyst 4500 series switch
increments in the same manner as in the previous scenario (bridge ID priority + VLAN number). If
the switch is in VLAN 1, the new bridge ID will be 32,769. Because 32,769 is greater than 32,768,
this switch cannot become the root switch. The network designates a new root switch; the spanning
tree topology also changes to reflect the new root switch.
If the bridge priority of the Catalyst 4006 has been lowered administratively and you use the same
configuration in the new Catalyst 4500 series switch, then the switch remains the root switch and
the spanning tree topology does not change.
For more information on migrating your supervisor engine from a Catalyst 4006 switch to a
Catalyst 4500 series switch, see the “Migrating a Supervisor Engine II from a Catalyst 4006 Switch to
a Catalyst 4500 Series Switch” section on page 28-10.
Understanding How MST Works
The Multiple Spanning Tree (MST) feature is the IEEE 802.1s and is an amendment to 802.1Q. MST
extends the 802.1w Rapid Spanning Tree (RST) algorithm to multiple spanning trees. This extension
provides for both rapid convergence and load balancing in a VLAN environment. The MST protocol is
currently being further developed; the MST feature for this release is based on a draft version of the IEEE
standard. The protocol, as implemented in this release, is backward compatible with 802.1D STP,
802.1w, the Rapid Spanning Tree Protocol (RSTP), and the Cisco PVST+ architecture.
MST allows you to build multiple spanning trees over VLAN trunks. You can group and associate
VLANs to spanning tree instances. Each instance can have a topology independent of other spanning
tree instances. This new architecture provides multiple forwarding paths for data traffic and enables load
balancing. Network fault tolerance is improved because a failure in one instance (forwarding path) does
not affect other instances (forwarding paths).
In large networks, having different VLAN-spanning tree instance assignments that are located in
different parts of the network makes it easier to administrate and utilize redundant paths. However, a
spanning tree instance can exist only on bridges that have compatible VLAN-instance assignments. MST
requires that you configure a set of bridges with the same MST configuration information, allowing them
to participate in a given set of spanning tree instances. Interconnected bridges that have the same MST
configuration are referred to as an MST region.
7-14
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Understanding How MST Works
MST uses the modified RSTP version called the Multiple Spanning Tree Protocol (MSTP). The MST
feature has these characteristics:
•
MST runs a variant of spanning tree called Internal Spanning Tree (IST). IST augments the Common
Spanning Tree (CST) information with internal information about the MST region. The MST region
appears as a single bridge to adjacent Single Spanning Tree (SST) and MST regions.
•
A bridge running MST provides interoperability with single spanning tree bridges as follows:
– MST bridges run a variant of STP (IST) that augments the Common Spanning Tree (CST)
information with internal information about the MST region.
– IST connects all the MST bridges in the region and appears as a subtree in the CST that
encompasses the whole bridged domain. The MST region appears as a virtual bridge to adjacent
SST bridges and MST regions.
– The collection of ISTs in each MST region, the CST that interconnects the MST regions, and
the SST bridges define Common and Internal Spanning Tree (CIST). CIST is the same as an IST
inside an MST region and the same as CST outside an MST region. The STP, RSTP, and MSTP
together elect a single bridge as the root of CIST.
•
MST establishes and maintains additional spanning trees within each MST region. These spanning
trees are referred to as MST instances (MSTIs). The IST is numbered 0, and the MSTIs are
numbered 1, 2, 3,... and so on. Any given MSTI is local to the MST region that is independent of
MSTIs in another region, even if the MST regions are interconnected. MST instances combine with
the IST at the boundary of MST regions to become the CST as follows:
– Spanning tree information for an MSTI is contained in an MSTP record (M-record).
M-records are always encapsulated within MST BPDUs (MST BPDUs). The original spanning
trees computed by MSTP are called M-trees. M-trees are active only within the MST region.
M-trees merge with the IST at the boundary of the MST region and form the CST.
•
MST provides interoperability with PVST+ by generating PVST+ BPDUs for the non-CST VLANs.
•
MST supports some of the PVST+ extensions in MSTP as follows:
– UplinkFast and BackboneFast are not available in MST mode; they are part of RSTP.
– PortFast is supported.
– BPDU filtering and BPDU guard are supported in MST mode.
– Loop guard and root guard are supported in MST. MST preserves the VLAN 1 disabled
functionality except that BPDUs are still transmitted in VLAN 1.
– MST switches behave as if MAC reduction is enabled.
– For private VLANs, secondary VLANs are mapped to the same instance as the primary.
Note the following guidelines when using MST:
•
Do not disable spanning tree on any VLAN in any of the PVST bridges.
•
Ensure that all PVST spanning tree root bridges have lower (numerically higher) priority than the
CST root bridge.
•
Do not use PVST bridges as the root of CST.
•
Ensure that trunks carry all of the VLANs that are mapped to an instance or do not carry any VLANs
at all.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-15
Chapter 7
Configuring Spanning Tree
Understanding How MST Works
•
Do not connect switches with access links because access links may partition a VLAN.
•
Any MST configuration involving a large number of either existing or new logical VLAN ports
should be carried out during the maintenance window. This action should be taken because the
complete MST database gets reinitialized for any incremental changes (such as adding new VLANs
to instances or moving VLANs across instances).
Rapid Spanning Tree Protocol
RSTP significantly reduces the time that it takes you to reconfigure the active topology of the network
when changes to the physical topology or its configurations parameters occur. RSTP selects one switch
as the root of a spanning-tree-connected active topology and assigns port roles to individual ports of the
switch, depending on whether that port is part of the active topology.
RSTP provides rapid connectivity following the failure of a switch, switch port, or a LAN. A new root
port and the designated port on the other side of the bridge transition to forwarding through an explicit
handshake between them. RSTP allows switch port configuration so that the ports can transition to
forwarding directly when the switch reinitializes.
RSTP, specified in 802.1w, supersedes STP, which is specified in 802.1D, while retaining compatibility
with STP. RSTP provides the structure on which the MST operates. You configure RSTP when you
configure the MST feature. For more information, see the “Configuring MST” section on page 7-46.
RSTP provides backward compatibility with 802.1D bridges, as follows:
•
RSTP selectively sends 802.1D-configured BPDUs and Topology Change Notification (TCN)
BPDUs on a per-port basis.
•
When a port initializes, the Migration Delay timer starts and RSTP BPDUs are transmitted. While
the Migration Delay timer is active, the bridge processes all BPDUs that are received on that port.
RSTP BPDUs are not visible on the port. Only version 3 BPDUs are visible on the port.
•
If the bridge receives an 802.1D BPDU after a port’s Migration Delay timer expires, the bridge
assumes that it is connected to an 802.1D bridge and starts using only 802.1D BPDUs.
•
When RSTP uses 802.1D BPDUs on a port and receives an RSTP BPDU after the migration delay
expires, RSTP restarts the Migration Delay timer and begins using RSTP BPDUs on that port.
RSTP Port Roles
RSTP uses the following definitions for port roles:
•
Root—A forwarding port that is elected for the spanning tree topology.
•
Designated—A forwarding port that is elected for every switched LAN segment.
•
Alternate—An alternate path to the root bridge to that provided by the current root port.
•
Backup—A backup for the path that is provided by a designated port toward the leaves of the
spanning tree. Backup ports can exist only where two ports are connected together in a loopback by
a point-to-point link or bridge with two or more connections to a shared LAN segment.
•
Disabled—A port that has no role within the operation of spanning tree.
Port roles are assigned as follows:
7-16
•
A root port or designated port role includes the port in the active topology.
•
An alternate port or backup port role excludes the port from the active topology.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Understanding How MST Works
RSTP Port States
The port state controls the forwarding and learning processes and provides the values of discarding,
learning, and forwarding. See Table 7-3 for a comparison between STP port states and RSTP port states.
Table 7-3
Comparison Between STP and RSTP Port States
Operational Status
STP Port State
RSTP Port State
1
Port Included in Active Topology?
2
Enabled
Blocking
Enabled
Listening
Discarding
No
Enabled
Learning
Learning
Yes
Enabled
Forwarding
Forwarding
Yes
Disabled
Disabled
Discarding
No
Discarding
No
1. IEEE 802.1D port state designation.
2. IEEE 802.1w port state designation. In this publication, discarding is the same as blocking in MST.
In a stable topology, RSTP ensures that every root port and designated port transition to forwarding while
all alternate ports and backup ports are always in the discarding state.
MST-to-SST Interoperability
A virtual bridged LAN may contain interconnected regions of SST and MST bridges. See Figure 7-8.
Network with Interconnected SST and MST Regions
MST
Region
B
r
B
F
B
B
F
r
SST
b
Region
F
F
F
r
b SST
Region
B
F
F
F/f = Forwarding
B/b = Blocking
R = Root Bridge
r = Root port
r
r
r
r
F
F
F
F
F
F
R
MST
Region
68285
Figure 7-8
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-17
Chapter 7
Configuring Spanning Tree
Understanding How MST Works
To the spanning tree protocol running in the SST region, an MST region appears as a single SST or
pseudobridge. Pseudobridges operate as follows:
•
The same values for root identifiers and root path costs are sent in all BPDUs of all the pseudobridge
ports. Pseudobridges differ from a single SST bridge as follows:
– The pseudobridge BPDUs have different bridge identifiers. This difference does not affect STP
operation in the neighboring SST regions because the root identifier and root cost are the same.
– BPDUs that are sent from the pseudobridge ports may have significantly different message ages.
Because the message age increases by 1 second for each hop, the difference in the message age
is in the order of seconds.
•
Data traffic from one port of a pseudobridge (a port at the edge of a region) to another port follows
a path that is entirely contained within the pseudobridge or MST region.
•
Data traffic belonging to different VLANs may follow different paths within the MST regions that
are established by MST.
•
Loop prevention is achieved by either of the following:
– Blocking the appropriate pseudobridge ports by allowing one forwarding port on the boundary
and blocking all other ports.
– Setting the CST partitions to block the ports of the SST regions.
•
A pseudobridge differs from a single SST bridge because the BPDUs that are sent from the
pseudobridge’s ports have different bridge identifiers. The root identifier and root cost are the same
for both bridges.
Common Spanning Tree
802.1Q specifies a single spanning tree for all the VLANs called CST. In a Catalyst 4500 series switch
running PVST+, the VLAN 1 spanning tree corresponds to CST. In a Catalyst 4500 series switch running
MST, IST (instance 0) corresponds to CST.
MST Instances
This release supports up to 16 instances; each spanning tree instance is identified by an instance ID that
ranges from 0 to 15. Instance 0 is mandatory and is always present. Instances 1 through 15 are optional.
MST Configuration
MST configuration has three parts as follows:
•
Name—A 32-character string (null padded and null terminated) identifying the MST region.
•
Revision number—An unsigned 16-bit number that increments each time that a change is made to
the configuration.
Note
7-18
You must set and update the revision number manually, because the number does not
auto-increment each time that you commit the MST configuration.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Understanding How MST Works
•
MST configuration table—An array of 4096 bytes. Each byte, interpreted as an unsigned integer,
corresponds to a VLAN. The value is the instance number to which the VLAN is mapped. The first
byte that corresponds to VLAN 0 and the 4096th byte that corresponds to VLAN 4095 are unused
and always set to zero.
You must configure each byte manually. You can use SNMP or the CLI to perform the configuration.
MST BPDUs contain the MST configuration ID and the checksum. An MST bridge accepts an MST
BPDU only if the MST BPDU configuration ID and the checksum match its own MST region
configuration ID and checksum. If one value is different, the MST BPDU is treated as an SST BPDU.
When you modify an MST configuration through either a console or Telnet connection, the session exits
without committing those changes and the edit buffer locks. Further configuration is impossible until
you discard the existing edit buffer and acquire a new edit buffer by entering the set spantree mst config
rollback force command.
MST Region
Interconnected bridges that have the same MST configuration are referred to as an MST region. There
is no limit on the number of MST regions in the network.
To form an MST region, bridges can be either of the following:
•
An MST bridge that is the only member of the MST region.
•
An MST bridge that is interconnected by a LAN. A LAN’s designated bridge has the same MST
configuration as an MST bridge. All the bridges on the LAN can process MST BPDUs.
If you connect two MST regions with different MST configurations, the MST regions do the following:
•
Load balance across redundant paths in the network. If two MST regions are redundantly connected,
all traffic flows on a single connection with the MST regions in a network.
•
Provide an RSTP handshake to enable rapid connectivity between regions. However, the
handshaking is not as fast as between two bridges. To prevent loops, all the bridges inside the region
must agree upon the connections to other regions. This situation introduces a certain delay. We do
not recommend partitioning the network into a large number of regions.
Boundary Ports
A port that connects an MST region to an SST region running RSTP (802.1w), an SST region running
STP (802.1D), or another MST region is a boundary port. A boundary port is a port that connects to a
LAN, the designated bridge of which, is either an SST bridge or a bridge with a different MST
configuration. A designated port knows that it is on the boundary if it detects an STP bridge or receives
an agreement message from an RST or MST bridge with a different configuration.
At the boundary, the role of MST ports do not matter; their state is forced to be the same as the IST port
state. If the boundary flag is set for the port, the MSTP Port Role selection mechanism assigns a port
role to the boundary and the same state as that of the IST port. The IST port at the boundary can take up
any port role except a backup port role.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-19
Chapter 7
Configuring Spanning Tree
Understanding How MST Works
IST Master
The IST master of an MST region is the bridge with the lowest bridge identifier and the least path cost
to the CST root. If an MST bridge is the root bridge for CST, then it is the IST master of that MST region.
If the CST root is outside the MST region, then one of the MST bridges at the boundary is selected as
the IST master. Other bridges on the boundary that belong to the same region eventually block the
boundary ports that lead to the root.
If two or more bridges at the boundary of the region have an identical path to the root, you can set a
slightly lower bridge priority to make a specific bridge the IST master.
The root path cost and message age inside a region stays constant, but the IST path cost is incremented
and the IST remaining hops is decremented at each hop. Enter the show spantree mst command to
display the information about the IST master, path cost, and remaining hops for the bridge.
Edge Ports
A port that is connected to a nonbridging device (for example, a host or a router) is an edge port. A port
that connects to a hub is also an edge port, provided that the hub or any LAN that is connected by it does
not have a bridge. These ports start forwarding as soon as the link is up.
MST requires that all ports are configured for each host or router. To establish rapid connectivity after a
failure, you need to block the nonedge-designated ports of an intermediate bridge. If the port connects
to another bridge that can send back an agreement, then the port starts forwarding immediately.
Otherwise, the port requires twice the forward delay time to start forwarding again. You must explicitly
configure the ports that are connected to the hosts and routers as edge ports while using MST.
Note
To configure a port as an edge port, you enable PortFast on that port. See Chapter 8, “Configuring
Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Guard.”
When you enter the show spantree portfast mod/port command, if the designation for a port is
displayed as edge, that port is also a PortFast port.
To prevent a misconfiguration, PortFast turns off operationally if the port receives a BPDU. You can
display the configured and operational status of PortFast by using the show spantree mst mod/port
command.
Link Type
You can establish rapid connectivity only on point-to-point links. For correct operation of the protocol,
you must explicitly configure ports to a host or router. However, cabling in most networks meets this
requirement, and you can avoid explicit configuration by treating all full-duplex links as point-to-point
links. Enter the set spantree mst link-type command to configure point-to-point links.
7-20
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Understanding How MST Works
Message Age and Hop Count
IST and MST instances do not use the Message Age and Maximum Age timer settings in the BPDU. IST
and MST use a separate hop count mechanism that is very similar to the IP TTL mechanism. You can
configure each MST bridge with a maximum hop count. The root bridge of the instance sends a BPDU
(or M-record) with the remaining hop count that is equal to the maximum hop count. When a bridge
receives a BPDU (or M-record), it decrements the received remaining hop count by one. The bridge
discards the BPDU (M-record) and ages out the information held for the port if the count reaches zero
after decrementing. The nonroot bridges propagate the decremented count as the remaining hop count in
the BPDUs (M-records) they generate.
The Message Age and Maximum Age timer settings in the RST portion of the BPDU remain the same
throughout the region, and the same values are propagated by the region’s designated ports at the
boundary.
MST-to-PVST+ Interoperability
These guidelines apply in a topology where you configure MST switches (all in the same region) to
interact with PVST+ switches that have VLANs 1–100 set up to span throughout the network:
•
Configure the root for all VLANs inside the MST region. The ports that belong to the MST switch
at the boundary simulate PVST+ and send PVST+ BPDUs for all the VLANs. This example shows
the ports simulating PVST:
Console> (enable) show spantree mst 3
Spanning tree mode
MST
Instance
3
VLANs Mapped:
31-40
Designated
Designated
Designated
Designated
Root
Root Priority
Root Cost
Root Port
Bridge ID MAC ADDR
Bridge ID Priority
Port
-----------------------6/1
Boundary(PVST)
6/2
Boundary(PVST)
00-10-7b-bb-2f-00
8195 (root priority:8192, sys ID ext:3)
0
Remaining Hops 20
1/0
00-10-7b-bb-2f-00
8195 (bridge priority:8192, sys ID ext:3)
State
Role Cost
Prio Type
------------- ---- -------- -----------------------forwarding
BDRY
10000
30 P2P,
blocking
BDRY
20000
32 P2P,
If you enable loop guard on the PVST+ switches, the ports might change to a loop-inconsistent state
when the MST switches change their configuration. To correct the loop-inconsistent state, you must
disable and reenable loop guard on that PVST+ switch.
•
Do not locate the root for some or all of the VLANs inside the PVST+ side of the MST switch,
because when the MST switch at the boundary receives PVST+ BPDUs for all or some of the
VLANs on its designated ports, root guard sets the port to the blocking state. Do not designate
switches with a slower CPU running PVST+ as a switch running MST.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-21
Chapter 7
Configuring Spanning Tree
Understanding How BPDU Skewing Works
When you connect a PVST+ switch to two different MST regions, the topology change from the PVST+
switch does not pass beyond the first MST region. In this case, the topology changes are only propagated
in the instance to which the VLAN is mapped. The topology change stays local to the first MST region
and the CAM entries in the other region are not flushed To make the topology change visible throughout
other MST regions, you can map that VLAN to IST or connect the PVST+ switch to the two regions
through access links.
Understanding How BPDU Skewing Works
BPDU skewing is the difference between when the BPDUs are expected to be received and the time
BPDUs are actually received. Skewing occurs when the following occurs:
•
Spanning tree timers lapse.
•
Expected BPDUs are not received.
•
Spanning tree detects topology changes.
The skew causes BPDUs to reflood the network to keep the spanning tree topology database current.
The root switch advertises its presence by sending out BPDUs for the configured Hello time interval.
The nonroot switches receive and process one BPDU during each configured time period. A VLAN
might not receive the BPDU as scheduled. If the BPDU is not received on a VLAN at the configured
time interval, the BPDU is skewed.
Spanning tree uses the Hello Time (see the “Configuring the Hello Time” section on page 7-44) to detect
when a connection to the root switch exists through a port and when that connection is lost. This feature
applies to both PVST+ and MISTP. In MISTP, the skew detection is on a per-instance basis.
BPDU skewing detects BPDUs that are not processed in a regular time frame on the nonroot switches in
the network. If BPDU skewing occurs, a syslog message is displayed. The syslog applies to both PVST+
and MISTP.
The number of syslog messages that are generated may impact the convergence of the network and the
CPU utilization of the switch. New syslog messages are not generated as individual messages for every
VLAN because the higher the number of syslog messages that are reported, the slower the switching
process will be. To reduce the impact on the switch, the syslog messages are as follows:
•
Generated 50 percent of the maximum age time (see the “Configuring the Maximum Aging Time”
section on page 45)
•
Rate limited at one for every 60 seconds
Using PVST+
PVST+ is the default spanning tree mode for Catalyst 4500 series switches. The following sections
describe how to configure PVST+ on Ethernet VLANs.
Default PVST+ Configuration
Table 7-4 shows the default PVST+ configuration.
7-22
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Using PVST+
Table 7-4
PVST+ Default Configuration
Feature
Default Value
VLAN 1
All ports assigned to VLAN 1
Enable state
PVST+ enabled for all VLANs
MAC address reduction
Disabled
Bridge priority
32,768
Bridge ID priority
32,769 (bridge priority plus system ID extension of VLAN 1)
Port priority
32
Port cost
•
Gigabit Ethernet: 4
•
Fast Ethernet: 10
•
FDDI/CDDI: 10
•
Ethernet: 100
Default spantree port cost
mode
Short (802.1D)
Port VLAN priority
Same as port priority but configurable on a per-VLAN basis in PVST+
Port VLAN cost
Same as port cost but configurable on a per-VLAN basis in PVST+
Maximum aging time
20 sec
Hello time
2 sec
Forward delay time
15 sec
Setting the PVST+ Bridge ID Priority
The bridge ID priority is the priority of a VLAN when the switch is in PVST+ mode.
When the switch is in PVST+ mode without MAC address reduction enabled, you can enter a bridge
priority value between 0–65,535. The VLAN bridge ID priority becomes that value.
When the switch is in PVST+ mode with MAC address reduction enabled, you can enter one of 16 bridge
priority values: 0, 4096, 8192, 12,288, 16,384, 20,480, 24,576, 28,672, 32,768, 36,864, 40,960, 45,056,
49,152, 53,248, 57,344, or 61,440.
The switch creates the bridge ID priority by combining the VLAN bridge priority with the system ID
extension (the ID of the VLAN).
To set the spanning tree bridge priority for a VLAN, perform this task in privileged mode:
Task
Command
Step 1
Set the bridge ID priority for a VLAN.
set spantree priority bridge_ID_priority [vlan]
Step 2
Verify the bridge ID priority.
show spantree [vlan] [active]
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-23
Chapter 7
Configuring Spanning Tree
Using PVST+
This example shows how to set the PVST+ bridge ID when MAC address reduction is not enabled
(default):
Console>
Spantree
Console>
VLAN 1
Spanning
Spanning
Spanning
(enable) set spantree priority 30000 1
1 bridge priority set to 30000.
(enable) show spantree 1
tree mode
tree type
tree enabled
PVST+
ieee
Designated Root
00-60-70-4c-70-00
Designated Root Priority
16384
Designated Root Cost
19
Designated Root Port
2/3
Root Max Age
14 sec
Hello Time 2 sec
Forward Delay 10 sec
Bridge ID MAC ADDR
Bridge ID Priority
Bridge Max Age 20 sec
Port
-----------------------1/1
1/2
2/1
2/2
00-d0-00-4c-18-00
30000
Hello Time 2 sec
Forward Delay 15 sec
Vlan
---1
1
1
1
Port-State
Cost
Prio Portfast Channel_id
------------- --------- ---- -------- ---------not-connected
4
32 disabled 0
not-connected
4
32 disabled 0
not-connected
100
32 disabled 0
not-connected
100
32 disabled 0
This example shows how to set the PVST+ bridge ID priority when MAC reduction is enabled:
Console> (enable) set spantree priority 32768 1
Spantree 1 bridge ID priority set to 32769
(bridge priority: 32768 + sys ID extension: 1)
Console> (enable) show spantree 1/1 1
VLAN 1
Spanning tree mode
PVST+
Spanning tree type
ieee
Spanning tree enabled
Designated Root
00-60-70-4c-70-00
Designated Root Priority
16384
Designated Root Cost
19
Designated Root Port
2/3
Root Max Age
14 sec
Hello Time 2 sec
Forward Delay 10 sec
Bridge ID MAC ADDR
Bridge ID Priority
Bridge Max Age 20 sec
Port
-----------------------1/1
1/2
2/1
2/2
7-24
00-d0-00-4c-18-00
32769 (bridge priority: 32768, sys ID ext: 1)
Hello Time 2 sec
Forward Delay 15 sec
Vlan
---1
1
1
1
Port-State
Cost
Prio Portfast Channel_id
------------- --------- ---- -------- ---------not-connected
4
32 disabled 0
not-connected
4
32 disabled 0
not-connected
100
32 disabled 0
not-connected
100
32 disabled 0
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Using PVST+
Configuring the PVST+ Port Cost
You can configure the port cost of switch ports. Ports with lower port costs are more likely to be chosen
to forward frames. Assign lower numbers to ports that are attached to faster media (such as full duplex),
and higher numbers to ports that are attached to slower media.The possible range of cost is from
1–65535. The default differs for different media. Typically, the path cost is 1000 ÷ LAN speed in
megabits per second.
To configure the port cost for a port, perform this task in privileged mode:
Task
Command
Step 1
Configure the port cost for a switch port.
set spantree portcost {mod/port} cost
Step 2
Verify the port cost setting.
show spantree mod/port
This example shows how to configure the port VLAN priority on a port and verify the configuration:
Console> (enable) set spantree portcost 2/3 12
Spantree port 2/3 path cost set to 12.
Console> (enable) show spantree 2/3
VLAN 1
.
.
Port
Vlan Port-State
Cost
Prio Portfast Channel_id
------------------------ ---- ------------- --------- ---- -------- ---------1/1
1
not-connected
4
32 disabled 0
1/2
1
not-connected
4
32 disabled 0
2/1
1
not-connected
100
32 disabled 0
2/2
1
not-connected
100
32 disabled 0
2/3
1
forwarding
12
32 disabled 0
2/4
1
not-connected
100
32 disabled
Configuring PVST+ Port Priority
You can configure the port priority of switch ports in PVST+ mode. The port with the lowest priority
value forwards frames for all VLANs. The possible port priority value is from 0–63. The default is 32.
If all ports have the same priority value, the port with the lowest port number forwards frames.
To configure the port priority for a port, perform this task in privileged mode:
Task
Command
Step 1
Configure the port priority for a switch port.
set spantree portpri mod_num/port_num priority
Step 2
Verify the port priority setting.
show spantree mod/port
This example shows how to configure the port priority for a port:
Console> (enable) set spantree portpri 2/3 16
Bridge port 2/3 port priority set to 16.
Console> (enable) show spantree 2/3
VLAN 1
.
.
.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-25
Chapter 7
Configuring Spanning Tree
Using PVST+
Port
-----------------------1/1
1/2
2/1
2/2
2/3
2/4
Vlan
---1
1
1
1
1
1
Port-State
Cost
Prio Portfast Channel_id
------------- --------- ---- -------- ---------not-connected
4
32 disabled 0
not-connected
4
32 disabled 0
not-connected
100
32 disabled 0
not-connected
100
32 disabled 0
forwarding
19
16 disabled 0
not-connected
100
32 disabled 0
Configuring the PVST+ Default Port Cost Mode
If any switch in your network is using a port speed of 10 Gb or over and the network is using PVST+
spanning tree mode, all switches in the network must have the same path cost defaults. You can enter the
set spantree defaultcostmode command to force all VLANs that are associated with all the ports to have
the same path cost default set.
There are two default port cost modes available–short and long.
•
The short mode has these parameters:
– Portcost.
– Portvlancost.
– When you enable UplinkFast, the actual cost is incremented by 3000.
•
The long mode has these parameters:
– Portcost.
– Portvlancost.
– When you enable UplinkFast, the actual cost is incremented by 10,000,000.
– EtherChannel computes the cost of a bundle using the formula,
AVERAGE_COST/NUM_PORT.
The default port cost mode in PVST+ is short. For port speeds of 10 Gb and greater, you must set the
default port cost mode to long.
To change the default port cost mode, perform this task in privileged mode:
Task
Command
Configure the default port cost mode.
set spantree defaultcostmode {short | long}
This example shows how to configure the default port cost mode:
Console> (enable) set spantree defaultcostmode long
Portcost and portvlancost set to use long format default values.
Console> (enable)
Configuring the PVST+ Port VLAN Cost
You can configure the port cost for a port on a per-VLAN basis. Ports with a lower port VLAN cost are
more likely to be chosen to forward frames. You should assign lower numbers to ports that are attached
to faster media (such as full duplex) and higher numbers to ports that are attached to slower media. The
default cost differs for different media.
You can set a cost value from 1– 65535.
7-26
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Using PVST+
To configure the port VLAN cost for a port, perform this task in privileged mode:
Task
Command
Configure the port VLAN cost for a
VLAN on a switch port.
set spantree portvlancost {mod/port} [cost cost] [vlan_list]
This example shows how to configure the port VLAN cost on a port:
Console> (enable) set spantree portvlancost 2/3 cost 20000 1-5
Port 2/3 VLANs 6-11,13-1005,1025-4094 have path cost 12.
Port 2/3 VLANs 1-5,12 have path cost 20000.
This parameter applies to trunking ports only.
Console> (enable
Configuring the PVST+ Port VLAN Priority
When the switch is in PVST+ mode, you can set the port priority for a trunking port in a VLAN. The
port with the lowest priority value for a specific VLAN forwards frames for that VLAN. The possible
port VLAN priority range is from 0–63. The default is 32. If all ports have the same priority value for a
particular VLAN, the port with the lowest port number forwards frames for that VLAN.
The port VLAN priority value must be lower than the port priority value.
To configure the port VLAN priority for a port, perform this task in privileged mode:
Task
Command
Step 1
Configure the port VLAN priority for a VLAN on set spantree portvlanpri mod_num/port_num
a switch port.
priority [vlans]
Step 2
Verify the port VLAN priority.
show config all
This example shows how to configure the port VLAN priority on a port:
Console> (enable) set spantree portvlanpri 2/3 16 6
Port 2/3 vlans 6 using portpri 16.
Port 2/3 vlans 1-5,7-800,802-1004,1006-4094 using portpri 32.
Port 2/3 vlans 801,1005 using portpri 4.
This parameter applies to trunking ports only.
Console> (enable) show config all
.
.
.
set spantree portcost
2/12,2/15 19
set spantree portcost
2/1-2,2/4-11,2/13-14,2/16-48 100
set spantree portcost
2/3 12
set spantree portpri
2/1-48 32
set spantree portvlanpri 2/1 0
set spantree portvlanpri 2/2 0
.
.
.
set spantree portvlanpri 2/48 0
set spantree portvlancost 2/1 cost 99
set spantree portvlancost 2/2 cost 99
set spantree portvlancost 2/3 cost 20000 1-5,12
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-27
Chapter 7
Configuring Spanning Tree
Using Rapid PVST+
Disabling the PVST+ Mode on a VLAN
When the switch is in PVST+ mode, you can disable spanning tree on individual VLANs or all VLANs.
When you disable spanning tree on a VLAN, the switch does not participate in spanning tree and any
BPDUs that are received in that VLAN are flooded on all ports.
Caution
Do not disable spanning tree on a VLAN unless all switches and bridges in the VLAN have spanning
tree disabled. You cannot disable spanning tree on some switches or bridges in a VLAN and leave it
enabled on other switches or bridges in the VLAN. Doing so can have unexpected results because
switches and bridges with spanning tree enabled will have incomplete information regarding the physical
topology of the network.
Caution
We do not recommend disabling spanning tree, even in a topology that is free of physical loops. Spanning
tree serves as a safeguard against misconfigurations and cabling errors. Do not disable spanning tree in
a VLAN without ensuring that there are no physical loops present in the VLAN.
To disable PVST+ mode, perform this task in privileged mode:
Task
Command
Disable PVST+ mode on a VLAN.
set spantree disable vlans [all]
This example shows how to disable PVST+ on a VLAN:
Console> (enable) set spantree disable 4
Spantree 4 disabled.
Console> (enable)
Using Rapid PVST+
To configure Rapid PVST+, you need to also configure PVST+ on your switch. You can configure
PVST+ either before or after you enable Rapid PVST+.
To configure Rapid PVST+, perform this task in privileged mode:
7-28
Task
Command
Step 1
Enable Rapid PVST+.
set spantree mode rapid-pvst+
Step 2
Set the link type to point-to-point mode for
the port.
set spantree link-type mod/port point-to-point
Step 3
If any port on the switch is connected to a
port on a PVST+ switch, check for any
legacy bridges on the port.
clear spantree detected-protocols mod/port
Step 4
Verify the Rapid PVST+ configuration.
show spantree vlan
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Using Rapid PVST+
This example shows how to configure Rapid PVST+:
Console> (enable) set spantree mode rapid-pvst+
Spantree mode set to RAPID-PVST+.
Console> (enable) set spantree link-type 3/1 point-to-point
Link type set to point-to-point on port 3/1.
Console> (enable) clear spantree detected-protocols 3/1
Spanning tree protocol detection forced on port 3/1
Console> (enable)
This example show how to verify the Rapid PVST+ configuration for VLAN 1. The first line in the
output displays the spanning tree mode:
Console> show spantree 1
Spanning tree mode
RAPID-PVST+
Spanning tree type
ieee
Spanning tree enabled.
.
.
.
Port
State
Role
Cost
------------ ----------- ------- ----6/1
forwarding ROOT
20000
Prio
---16
Type
----------------Shared, PEER(STP)
Console>
This example shows how to verify the link type, edge port, and guard type for port 3/6:
Console> show spantree 3/6
Port 3/6
Edge Port:
No, (Configured) Default
Port Guard:
Default
Link Type:
P2P(Configured) Auto
Port
-----3/6
3/6
3/6
3/6
3/6
3/6
3/6
3/6
3/6
3/6
3/6
3/6
3/6
3/6
3/6
3/6
3/6
3/6
3/6
VLAN
----1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
State
---------listening
listening
listening
listening
listening
listening
listening
listening
listening
listening
listening
listening
listening
listening
listening
listening
listening
listening
listening
Role
-----DESG
DESG
DESG
DESG
DESG
DESG
DESG
DESG
DESG
DESG
DESG
DESG
DESG
DESG
DESG
DESG
DESG
DESG
DESG
Cost
-------20000
20000
20000
20000
20000
20000
20000
20000
20000
20000
20000
20000
20000
20000
20000
20000
20000
20000
20000
Prio
---32
32
32
32
32
32
32
32
32
32
32
32
32
32
32
32
32
32
32
Type
----P2P
P2P
P2P
P2P
P2P
P2P
P2P
P2P
P2P
P2P
P2P
P2P
P2P
P2P
P2P
P2P
P2P
P2P
P2P
Console>
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-29
Chapter 7
Configuring Spanning Tree
Using MISTP-PVST+ or MISTP
Using MISTP-PVST+ or MISTP
The default spanning tree mode on the Catalyst 4500 series switches is PVST+ mode. If you want to use
MISTP mode in your network, we recommend that you carefully follow the procedures that are described
in the following sections in order to avoid loss of connectivity in your network.
When you change the spanning tree mode from one mode to another, the current mode stops, the information
collected at run-time is used to build the port database for the new mode, and the new spanning tree mode
restarts the computation of the active topology. Information about the port states is lost; however, all of the
configuration parameters are preserved for the previous mode. If you return to the previous mode, the
configuration will still be there.
Note
We recommend that if you wish to use MISTP mode, you should configure all of your Catalyst 4500
series switches to run MISTP.
To use MISTP mode, you first enable an MISTP instance, and then map at least one VLAN to the
instance. You must have at least one forwarding port in the VLAN for the MISTP instance to be active.
If you are changing a switch from PVST+ mode to MISTP mode and you have other switches in the
network that are using PVST+, you must first enable MISTP-PVST+ mode on each switch on which you
intend to use MISTP so that PVST+ BPDUs can flow through the switches while you configure them.
When all switches in the network are configured in MISTP-PVST+, you can then enable MISTP on all
of the switches.
Default MISTP Mode Configuration
Table 7-5 shows the default configuration for MISTP and MISTP-PVST+ modes.
Table 7-5
MISTP Mode Default Configuration
Feature
Default Value
Enable state
Disabled until a VLAN is mapped to an MISTP instance
MAC address reduction
Disabled
Bridge priority
32,768
Bridge ID priority
32,769 (bridge priority plus the system ID extension of MISTP instance 1)
Port priority
32 (global)
Port cost
7-30
•
Gigabit Ethernet: 4
•
Fast Ethernet: 10
•
FDDI/CDDI: 10
•
Ethernet: 100
Default port cost mode
Short (802.1D)
Port VLAN priority
Same as port priority but configurable per VLAN in PVST+
Port VLAN cost
Same as port cost but configurable per VLAN in PVST+
Maximum aging time
20 sec
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Using MISTP-PVST+ or MISTP
Table 7-5
MISTP Mode Default Configuration (continued)
Feature
Default Value
Hello time
2 sec
Forward delay time
15 sec
Setting the MISTP-PVST+ Mode or MISTP Mode
If you enable MISTP in a PVST+ network, you must be very careful to avoid bringing down the network.
This section explains how to enable MISTP or MISTP-PVST+ on your network.
Caution
If you have more than 4500 VLAN ports that are configured on your switch, your network could crash
if you change from MISTP to either PVST+ or MISTP-PVST+ mode. To avoid losing connectivity,
reduce the number of configured VLAN ports on your switch to no more than 4500.
Caution
If you are working from a Telnet connection to your switch, the first time that you enable MISTP-PVST+
or MISTP mode, you must do so from the switch console. Do not use a Telnet connection through the
data port or you will lose the connection to the switch. Once you map a VLAN to an MISTP instance,
you can Telnet to the switch.
To change from PVST+ to MISTP-PVST+ or MISTP, perform this task in privileged mode:
Task
Command
Set a spanning tree mode.
set spantree mode {mistp | pvst+ | mistp-pvst+}
This example shows how to set a switch to MISTP-PVST+ mode:
Console> (enable) set spantree mode mistp-pvst+
PVST+ database cleaned up.
Spantree mode set to MISTP-PVST+.
Warning!! There are no VLANs mapped to any MISTP instance.
Console> (enable)
You can display VLAN-to-MISTP instance mapping information that is propagated from the root switch
at runtime. This display is available only in the MISTP or MISTP-PVST+ mode. When you are in
PVST+ mode, use the optional keyword config, to display the list of mappings that are configured on
the local switch.
Note
MAC addresses are not displayed when you specify the keyword config.
To display spanning tree mapping, perform this task in privileged mode:
Task
Command
Step 1
Set spanning tree mode to MISTP.
set spantree mode mistp
Step 2
Show spanning tree mapping.
show spantree mapping [config]
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-31
Chapter 7
Configuring Spanning Tree
Using MISTP-PVST+ or MISTP
This example shows how to display the spanning tree VLAN instance mapping in MISTP mode:
Console> (enable) set spantree mode mistp
PVST+ database cleaned up.
Spantree mode set to MISTP.
Console> (enable) show spantree mapping
Inst Root Mac
Vlans
---- ----------------- -------------------------1
00-50-3e-78-70-00 1
2
00-50-3e-78-70-00 3
00-50-3e-78-70-00 4
00-50-3e-78-70-00 5
00-50-3e-78-70-00 6
00-50-3e-78-70-00 7
00-50-3e-78-70-00 8
00-50-3e-78-70-00 9
00-50-3e-78-70-00 10
00-50-3e-78-70-00 11
00-50-3e-78-70-00 12
00-50-3e-78-70-00 13
00-50-3e-78-70-00 14
00-50-3e-78-70-00 15
00-50-3e-78-70-00 16
00-50-3e-78-70-00 -
Configuring the MISTP Bridge ID Priority
You can set the bridge ID priority for an MISTP instance when the switch is in MISTP or MISTP-PVST+
mode.
The switch combines the bridge priority value with the system ID extension (the ID of the MISTP
instance) to create the bridge ID priority. You can set 16 possible bridge priority values: 0, 4096, 8192,
12,288, 16,384, 20,480, 24,576, 28,672, 32,768, 36,864, 40,960, 45,056, 49,152, 53,248, 57,344, and
61,440.
To configure the bridge ID priority for an MISTP instance, perform this task in privileged mode:
Task
Command
Step 1
Configure the bridge ID priority for an
MISTP instance.
set spantree priority bridge_ID_priority [mistp-instance
instance]
Step 2
Verify the bridge ID priority.
show spantree mistp-instance instance [mod/port] active
The example shows how to configure the bridge ID priority for an MISTP instance:
Console> (enable) set spantree priority 32768 mistp-instance 1
Spantree 1 bridge ID priority set to 32769
(bridge priority: 32768 + sys ID extension: 1)
Console> (enable) show spantree mistp-instance 1
Instance 1
Spanning tree mode
MISTP
Spanning tree type
ieee
Spanning tree instance enabled
Designated Root
Designated Root Priority
Designated Root Cost
Designated Root Port
VLANs mapped:
7-32
00-05-31-40-64-00
32769 (root priority:32768, sys ID ext:1)
20000
1/1
1,74
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Using MISTP-PVST+ or MISTP
Root Max Age
20 sec
Bridge ID MAC ADDR
Bridge ID Priority
VLANs mapped:
Bridge Max Age 20 sec
Hello Time 2
sec
Forward Delay 15 sec
00-d0-02-27-9c-00
32769 (bridge priority:32768, sys ID ext:1)
1,74
Hello Time 2 sec
Forward Delay 15 sec
Port
-----------------------1/1
3/1
3/25
3/26
3/27
3/28
3/29
3/30
7/1-4
7/5
7/6
8/37
8/38
15/1
16/1
Console> (enable)
Inst
---1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Port-State
Cost
Prio Portfast Channel_id
------------- --------- ---- -------- ---------forwarding
20000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
blocking
5000
32 disabled 833
forwarding
20000
32 disabled 0
forwarding
20000
32 disabled 0
blocking
200000
32 disabled 0
blocking
200000
32 disabled 0
forwarding
20000
32 enabled 0
forwarding
20000
32 enabled 0
Configuring the MISTP Port Cost
You can configure the port cost of switch ports. When forwarding frames, the switch is more likely to
use ports with lower port costs. Assign lower numbers to ports that are attached to faster media (such as
full duplex) and higher numbers to ports that are attached to slower media.The possible range is from
1–65,535. The default differs for different media. Path cost is typically equal to 1000 ÷ LAN speed in
megabits per second.
To configure the port cost for a port, perform this task in privileged mode:
Task
Command
Step 1
Configure the port cost for a switch port.
set spantree portcost mod_num/port_num cost
Step 2
Verify the port cost setting.
show spantree mistp-instance instance
[mod_num/port_num] active
This example shows how to configure the port cost on an MISTP instance and verify the configuration:
Console>
Spantree
Console>
Instance
Spanning
Spanning
Spanning
(enable) set spantree portcost 1/1 20000
port 1/1 path cost set to 20000.
(enable) show spantree mistp-instance 1 active
1
tree mode
MISTP
tree type
ieee
tree instance enabled
Designated Root
00-05-31-40-64-00
Designated Root Priority
32769 (root priority:32768, sys ID ext:1)
Designated Root Cost
20000
Designated Root Port
1/1
VLANs mapped:
1,74
Root Max Age
20 sec
Hello Time 2 sec
Forward Delay 15 sec
Bridge ID MAC ADDR
00-d0-02-27-9c-00
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-33
Chapter 7
Configuring Spanning Tree
Using MISTP-PVST+ or MISTP
Bridge ID Priority
VLANs mapped:
Bridge Max Age 20 sec
Port
-----------------------1/1
3/1
3/25
3/26
3/27
3/28
3/29
3/30
7/1-4
7/5
7/6
8/37
8/38
15/1
16/1
Console> (enable)
32769 (bridge priority:32768, sys ID ext:1)
1,74
Hello Time 2 sec
Forward Delay 15 sec
Inst
---1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Port-State
Cost
Prio Portfast Channel_id
------------- --------- ---- -------- ---------forwarding
20000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
blocking
5000
32 disabled 833
forwarding
20000
32 disabled 0
forwarding
20000
32 disabled 0
blocking
200000
32 disabled 0
blocking
200000
32 disabled 0
forwarding
20000
32 enabled 0
forwarding
20000
32 enabled 0
Configuring the MISTP Port Priority
You can configure the port priority of switch ports. The port with the lowest priority value forwards
frames for all VLANs. The possible port priority value is from 0–63; the default is 32. If all ports have
the same priority value, the port with the lowest port number forwards frames.
To configure the port priority for a port, perform this task in privileged mode:
Task
Command
Step 1
Configure the port priority for a switch port.
set spantree portpri mod_num/port_num priority
[instance]
Step 2
Verify the port priority setting.
show spantree mistp-instance instance
[mod_num/port_num] active
This example shows how to configure the port priority and verify the configuration:
Console> (enable) set spantree portpri 1/1 32
Bridge port 1/1 port priority set to 32.
Console> (enable) show spantree mistp-instance 1
Instance 1
Spanning tree mode
MISTP
Spanning tree type
ieee
Spanning tree instance enabled
Designated Root
00-05-31-40-64-00
Designated Root Priority
32769 (root priority:32768, sys ID ext:1)
Designated Root Cost
20000
Designated Root Port
1/1
VLANs mapped:
1,74
Root Max Age
20 sec
Hello Time 2 sec
Forward Delay 15 sec
Bridge ID MAC ADDR
Bridge ID Priority
VLANs mapped:
Bridge Max Age 20 sec
7-34
00-d0-02-27-9c-00
32769 (bridge priority:32768, sys ID ext:1)
1,74
Hello Time 2 sec
Forward Delay 15 sec
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Using MISTP-PVST+ or MISTP
Port
-----------------------1/1
3/1
3/25
3/26
3/27
3/28
3/29
3/30
7/1-4
7/5
7/6
8/37
8/38
15/1
16/1
Console> (enable)
Inst
---1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Port-State
Cost
Prio Portfast Channel_id
------------- --------- ---- -------- ---------forwarding
20000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
forwarding
200000
32 disabled 0
blocking
5000
32 disabled 833
forwarding
20000
32 disabled 0
forwarding
20000
32 disabled 0
blocking
200000
32 disabled 0
blocking
200000
32 disabled 0
forwarding
20000
32 enabled 0
forwarding
20000
32 enabled 0
Configuring the MISTP Port Instance Cost
You can configure the port instance cost for an instance of MISTP or MISTP-PVST+. Ports with a lower
instance cost are more likely to be chosen to forward frames. You should assign lower numbers to ports
that are attached to faster media (such as full duplex) and higher numbers to ports that are attached to
slower media. The default cost differs for different media. The possible value for port instance cost is
from 1–268435456.
To configure the port instance cost for a port, perform this task in privileged mode:
Task
Command
Configure the port instance cost on a
switch port.
set spantree portinstancecost {mod/port} [cost cost]
[instances]
This example shows how to configure the MISTP port instance cost on a port:
Console> (enable) set spantree portinstancecost 1/1 cost 110110 2
Port 1/1 instances 1,3-16 have path cost 20000.
Port 1/1 instances 2 have path cost 110110.
This parameter applies to trunking ports only.
Console> (enable)
Configuring the MISTP Port Instance Priority
You can set the port priority for an instance of MISTP. The port with the lowest priority value for a
specific MISTP instance forwards frames for that instance. The possible port instance range is from
0–63. If all ports have the same priority value for an MISTP instance, the port with the lowest port
number forwards frames for that instance.
To configure the port instance priority on an MISTP instance, perform this task in privileged mode:
Task
Command
Configure the port instance priority on an MISTP
instance.
set spantree portinstancepri {mod/port}
priority [instances]
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-35
Chapter 7
Configuring Spanning Tree
Using MISTP-PVST+ or MISTP
This example shows how to configure the port instance priority on an MISTP instance and verify the
configuration:
Console>
Port 1/1
Port 1/1
Console>
(enable) set spantree portinstancepri 1/1 16 2
MISTP Instances 2 using portpri 16.
mistp-instance 1,3-16 using portpri 32.
(enable)
Enabling an MISTP Instance
You can enable up to 16 MISTP instances. Each MISTP instance defines a unique spanning tree
topology. MISTP instance 1, the default instance, is enabled by default; however, you must map a VLAN
to it in order for it to be active. You can enable a single MISTP instance, a range of instances, or all
instances at once using the all keyword.
Note
The software does not display the status of an MISTP instance until it has a VLAN with an active port
mapped to it.
To enable an MISTP instance, perform this task in privileged mode:
Task
Command
Step 1
Enable an MISTP instance.
set spantree enable mistp-instance instance [all]
Step 2
Verify that the instance is enabled.
show spantree mistp-instance [instance] [active]
mod/port
Note
Enter the active keyword to display active ports only.
This example shows how to enable an MISTP instance:
Console> (enable) set spantree enable mistp-instance 2
Spantree 2 enabled.
Console>
Instance
Spanning
Spanning
Spanning
.
.
.
(enable) show spantree mistp-instance 2
2
tree mode
MISTP
tree type
ieee
tree instance enabled
Mapping VLANs to an MISTP Instance
When you are using MISTP-PVST+ or MISTP on a switch, you must map at least one VLAN to an
MISTP instance in order for MISTP-PVST+ or MISTP to be active.
Note
7-36
See Chapter 10, “Configuring VLANs” for details on using and configuring VLANs.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Using MISTP-PVST+ or MISTP
•
You can only map Ethernet VLANs to MISTP instances.
•
At least one VLAN in the instance must have an active port in order for MISTP-PVST+ or MISTP
to be active.
•
You can map as many Ethernet VLANs as you wish to an MISTP instance.
•
You cannot map a VLAN to more than one MISTP instance.
To map a VLAN to an MISTP instance, perform this task in privileged mode:
Task
Command
Step 1
Map a VLAN to an MISTP instance.
set vlan vlan mistp-instance instance
Step 2
Verify that the VLAN is mapped.
show spantree mistp-instance [instance] [active]
mod/port
This example shows how to map a VLAN to an MISTP instance 1 and verify the mapping:
Console> (enable) set vlan 6 mistp-instance 1
Vlan 6 configuration successful
Console> (enable) show spantree mist-instance 1
Instance 1
Spanning tree mode
MISTP-PVST+
Spanning tree type
ieee
Spanning tree instance enabled
Designated Root
00-d0-00-4c-18-00
Designated Root Priority
49153 (root priority: 49152, sys ID ext: 1)
Designated Root Cost
0
Designated Root Port
none
VLANs mapped:
6
Root Max Age
20 sec
Hello Time 2 sec
Forward Delay 15 sec
Bridge ID MAC ADDR
Bridge ID Priority
VLANs mapped:
Bridge Max Age 20 sec
00-d0-00-4c-18-00
49153 (bridge priority: 49152, sys ID ext: 1)
6
Hello Time 2 sec
Forward Delay 15 sec
Port
Inst Port-State
Cost
Prio Portfast Channel_id
------------------------ ---- ------------- --------- ---- -------- ---------2/12
1
forwarding
22222222
40 disabled 0
Determining an MISTP Instance—VLAN Mapping Conflicts
A VLAN can only be mapped to one MISTP instance. If you attempt to map a VLAN to more than one
instance, all of its ports are set to blocking mode. You can use the show spantree conflicts command to
determine to which MISTP instances you have attempted to map the VLAN.
This command prints a list of the MISTP instances that are associated with the VLAN, the MAC
addresses of the root switches that are sending the BPDUs containing the VLAN mapping information,
and the timers that are associated with the mapping of a VLAN to an MISTP instance. When only one
entry is printed or when all the entries are associated to the same instance, the VLAN is mapped to that
instance. If two or more entries in the list are associated with different MISTP instances, the VLAN is
in conflict.
To clear up the conflict, you must manually remove the incorrect mapping(s) from the root switch. The
remaining entry on the list becomes the official mapping.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-37
Chapter 7
Configuring Spanning Tree
Using MISTP-PVST+ or MISTP
To determine VLAN mapping conflicts, perform this task in privileged mode:
Task
Command
Determine VLAN mapping conflicts.
show spantree conflicts vlan
This example shows that there is an attempt to map VLAN 2 to MISTP instance 1 and to MISTP
instance 3 on two different switches as seen from a third switch in the topology:
Console> (enable) show
Inst MAC
---- ----------------1
00-30-a3-4a-0c-00
3
00-30-f1-e5-00-01
spantree conflicts 2
Delay
Time left
--------- --------inactive
20
inactive
10
The Delay timer shows the time in seconds remaining before the VLAN will join the instance. The field
displays inactive if the VLAN is already mapped to an instance (the timer has expired), or the VLAN is
in conflict between instances.
The Time Left timer shows the time in seconds left before the entry will expire and be removed from the
table. The timer is restarted every time an incoming BPDU confirms the mapping. Entries pertaining to
the root switch show inactive on the root switch itself.
The following examples are with VTP version 3 enabled. The root switch is also the primary server for
the nonroot switch. The root switch is not the primary server for the switch in conflict, because that
switch has been partitioned.
This example is from the root switch:
Console> (enable) show spantree conflicts 1
No conflicts for vlan 1.
Inst MAC
Delay
Time left
---- ----------------- --------- --------1 00-05-31-40-64-00 inactive inactive
Console> (enable)
This example is from the nonroot switch:
Console> (enable) show spantree conflicts 3
No conflicts for vlan 3.
Inst MAC
Delay
Time left
---- ----------------- --------- --------3 00-05-31-40-64-00 inactive
19
Console> (enable)
This example is from the switch in conflict (note that the switch is inactive):
Console> (enable) show spantree conflicts 6
Inst MAC
Delay
Time left
---- ----------------- --------- --------6 00-05-31-40-64-00 inactive
18
5 00-09-7b-62-b0-80 inactive inactive
Console> (enable)
7-38
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Configuring a Root Switch
Unmapping VLANs from an MISTP Instance
The keyword none is used to unmap the specified VLANs from the MISTP instances to which they are
currently mapped. When you unmap a VLAN from an MISTP instance, the resulting state of all the ports
of the VLAN (if the VLAN exists) is blocking.
To unmap a VLAN or all VLANs from an MISTP instance, perform this task in privileged mode:
Task
Command
Unmap a VLAN from an MISTP instance.
set vlan vlan mistp-instance none
This example shows how to unmap a VLAN from an MISTP instance:
Console> (enable) set vlan 6 mistp none
Vlan 6 configuration successful
Disabling MISTP-PVST+ or MISTP
When the switch is in MISTP mode, you disable spanning tree on an instance, not for the whole switch.
When you disable spanning tree on an MISTP instance, the instance still exists on the switch, all of the
VLANs mapped to it have all of their ports forwarding, and the instance BPDUs are flooded.
To disable an MISTP instance, perform this task in privileged mode:
Task
Command
Disable an MISTP instance.
set spantree disable mistp-instance instance [all]
This example shows how to disable an MISTP instance:
Console> (enable) set spantree disable mistp-instance 2
MI-STP instance 2 disabled.
Configuring a Root Switch
This section explains how to configure a primary root switch and a secondary root switch, and how you
can prevent a switch from becoming a root switch by using root guard.
Configuring a Primary Root Switch
You can set a root switch on a VLAN when the switch is in PVST+ mode or on an MISTP instance when
the switch is in MISTP mode. Enter the set spantree root command to lower the bridge priority (the
value that is associated with the switch) below the default (32,768); the switch can then become the root
switch.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-39
Chapter 7
Configuring Spanning Tree
Configuring a Root Switch
When you specify a switch as the primary root, the default bridge priority is modified so that it becomes
the root for the specified VLANs. Set the bridge priority to 8192. If this setting does not result in the
switch becoming a root, modify the bridge priority to be 1 less or the same as the bridge priority of the
current root switch. Because different VLANs could potentially have different root switches, the bridge
VLAN-priority chosen makes this switch the root for all the VLANs that are specified. If reducing the
bridge priority as low as 1 still does not make the switch the root switch, the system displays a message.
Caution
Enter the set spantree root command on backbone switches or distribution switches only, not on access
switches.
To configure a switch as the primary root switch, perform this task in privileged mode:
Task
Command
Configure a switch as the primary root switch.
set spantree root [vlans] [dia network_diameter]
[hello hello_time]
This example shows how to configure the primary root switch for VLANs 1–10:
Console> (enable)
VLANs 1-10 bridge
VLANs 1-10 bridge
VLANs 1-10 bridge
VLANs 1-10 bridge
Switch is now the
Console> (enable)
set spantree root 1-10 dia 4
priority set to 8192
max aging time set to 14 seconds.
hello time set to 2 seconds.
forward delay set to 9 seconds.
root switch for active VLANs 1-6.
To configure a switch as the primary root switch for an instance, perform this task in privileged mode:
Task
Command
Configure a switch as the primary root switch for set spantree root mistp-instance instance [dia
an instance.
network_diameter] [hello hello_time]
This example shows how to configure the primary root for an instance:
Console> (enable) set spantree root mistp-instance 2-4 dia 4
Instances 2-4 bridge priority set to 8192
VLInstances 2-4 bridge max aging time set to 14 seconds.
Instances 2-4 bridge hello time set to 2 seconds.
Instances 2-4 bridge forward delay set to 9 seconds.
Switch is now the root switch for active Instances 1-6.
Console> (enable)
Configuring a Secondary Root Switch
You can set a secondary root switch on a VLAN when the switch is in PVST+ mode or on an MISTP
instance when the switch is in MISTP mode.
The set spantree root secondary command reduces the bridge priority to 16,384, making it the probable
candidate to become the root switch if the primary root switch fails. You can run this command on more
than one switch to create multiple backup switches in case the primary root switch fails.
7-40
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Configuring a Root Switch
To configure a switch as the secondary root switch, perform this task in privileged mode:
Task
Command
Configure a switch as the secondary root switch.
set spantree root [secondary] vlans [dia
network_diameter] [hello hello_time]
This example shows how to configure the secondary root switch for VLANs 22 and 24:
Console> (enable) set spantree root secondary 22,24 dia 5 hello 1
VLANs 22,24 bridge priority set to 16384.
VLANs 22,24 bridge max aging time set to 10 seconds.
VLANs 22,24 bridge hello time set to 1 second.
VLANs 22,24 bridge forward delay set to 7 seconds.
Console> (enable)
To configure a switch as the secondary root switch for an instance, perform this task in privileged mode:
Task
Command
Configure a switch as the secondary root switch set spantree root [secondary] mistp-instance
for an instance.
instance [dia network_diameter] [hello hello_time]
This example shows how to set the secondary root for an instance:
Console> (enable) set spantree root secondary mistp-instance 2-4 dia 4
Instances 2-4 bridge priority set to 8192
VLInstances 2-4 bridge max aging time set to 14 seconds.
Instances 2-4 bridge hello time set to 2 seconds.
Instances 2-4 bridge forward delay set to 9 seconds.
Switch is now the root switch for active Instances 1-6.
Console> (enable)
Configuring a Root Switch to Improve Convergence
You can configure the root switch to speed up STP convergence time by reducing the value of the Hello
Time, Forward Delay Timer, and Maximum Age Timer parameters. For more information, see the
“Configuring Spanning Tree Timers” section on page 7-44.
Note
Reducing the timer parameters is possible only if your network has LAN links of 10 Mbps or faster. In
a network with links of 10 Mbps or faster, the network diameter can reach the maximum value of 7. With
WAN connections, you cannot reduce the parameters.
When a link failure occurs in a bridged network, network reconfiguration is not immediate.
Reconfiguring the default parameters (that are specified by IEEE 802.1D) for the Hello Time, Forward
Delay Timer, and Maximum Age Timer requires a 50-second delay. This reconfiguration delay depends
on the network diameter, which is the maximum number of bridges between any two end stations.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-41
Chapter 7
Configuring Spanning Tree
Configuring a Root Switch
To speed up convergence, use nondefault parameters values that are permitted by 802.1D. The
nondefault parameters for a reconvergence of 14 seconds are as follows:
Parameter
Time
Network Diameter (dia)
2 hops
Hello Time
2 sec
Forward Delay Timer
4 sec
Maximum Age Timer
6 sec
You can set these parameters on the Catalyst 4500 series switches without modifying the switches.
Note
You can set switch ports for improved convergence in PortFast mode. This setting affects only the
transition from disable (link down) to enable (link up), moving the port immediately to the forwarding
state. If a port in PortFast mode begins blocking, then it goes through listening and learning before
reaching the forwarding state.
To configure the spanning tree bridge to improve convergence, perform this task in privileged mode:
Task
Command
Step 1
Configure the Hello time for a VLAN or
MISTP instance.
set spantree hello interval [vlan] mistp-instance
[instances]
Step 2
Verify the configuration.
show spantree [vlan | mistp-instance instances]
Step 3
Configure the forward delay time for a
VLAN or MISTP instance.
set spantree fwddelay delay [vlan] mistp-instance
[instances]
Step 4
Verify the configuration.
show spantree [mod/port] mistp-instance [instances]
[active]
Step 5
Configure the maximum aging time for a
VLAN or MISTP instance.
set spantree maxage agingtime [vlans] mistp-instance
instances
Step 6
Verify the configuration.
show spantree [mod/port] mistp-instance [instances]
[active]
This example shows how to configure the spanning tree Hello Time, Forward Delay Timer, and
Maximum Age Timer to 2, 4, and 6 seconds:
Console>
Spantree
Console>
Console>
Spantree
Console>
Console>
Spantree
Console>
7-42
(enable) set spantree hello 2 100
100 hello time set to 7 seconds.
(enable)
(enable) set spantree fwddelay 4 100
100 forward delay set to 21 seconds.
(enable)
(enable) set spantree maxage 6 100
100 max aging time set to 36 seconds.
(enable)
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Configuring a Root Switch
Console> (enable)
VLANs 1-10 bridge
VLANs 1-10 bridge
VLANs 1-10 bridge
VLANs 1-10 bridge
Switch is now the
Console> (enable)
set spantree root 1-10 dia 4
priority set to 8192
max aging time set to 14 seconds.
hello time set to 2 seconds.
forward delay set to 9 seconds.
root switch for active VLANs 1-6.
Using Root Guard—Preventing Switches from Becoming Root
You may want to prevent switches from becoming the root switch. The root guard feature forces a port
to become a designated port so that no switch on the other end of the link can become a root switch.
When you enable root guard on a per-port basis, it is automatically applied to all of the active VLANs to
which that port belongs. When you disable root guard, it is disabled for the specified port(s). If a port goes
into the root-inconsistent state, it automatically goes into the listening state.
To prevent switches from becoming root, perform this task in privileged mode:
Task
Command
Step 1
Enable root guard on a port.
set spantree guard {root | none} mod/port
Step 2
Verify that root guard is enabled.
show spantree guard {mod/port | vlan} {mistp-instance
instance | mod/port}
Displaying Spanning Tree BPDU Statistics
Enter the show spantree statistics bpdu command to display the total number of spanning tree BPDUs
(transmitted, received, processed, and dropped). The command also provides the rate of the BPDUs in
seconds. The BPDU counters are cleared using the clear spantree statistics bpdu command or when
the system is booted.
To display spanning tree BPDU statistics, perform this task in normal mode (clear the statistics from
privileged mode):
Task
Command
Step 1
Display spanning tree BPDU statistics.
show spantree statistics bpdu
Step 2
Clear the BPDU statistics.
clear spantree statistics bpdu
This example shows how to display spanning tree BPDU statistics:
Console> show spantree statistics bpdu
Transmitted
Received
Processed
Dropped
-------------- -------------- -------------- -------------Total
Rate(/sec)
52943073
52016589
52016422
167
989
971
971
0
This example shows how to clear spanning tree BPDU statistics:
Console> (enable) clear spantree statistics bpdu
Spanning tree BPDU statistics cleared on the switch.
Console> (enable)
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-43
Chapter 7
Configuring Spanning Tree
Configuring Spanning Tree Timers
Configuring Spanning Tree Timers
Spanning tree timers affect the spanning tree performance. You can configure the spanning tree timers
for a VLAN in PVST+ or an MISTP instance in MISTP mode. If you do not specify a VLAN when the
switch is in PVST+ mode, VLAN 1 is assumed. If you do not specify an MISTP instance when the switch
is in MISTP mode, MISTP instance 1 is assumed.
Caution
Exercise care using these commands. For most situations, we recommend that you use the set spantree
root and set spantree root secondary commands to modify the spanning tree performance parameters.
Table 7-6 describes the switch variables that affect spanning tree performance.
Table 7-6
Switch Variable Descriptions
Variable
Description
Default
Hello Time
Determines how often the switch broadcasts its Hello message to
other switches.
20 sec
Maximum Age
Timer
2 sec
Measures the age of the received protocol information that is
recorded for a port and ensures that this information is discarded
when its age limit exceeds the value of the maximum age parameter
that is recorded by the switch. The timeout value is the maximum
age parameter of the switches.
Forward Delay
Timer
Monitors the time that is spent by a port in the learning and
listening states. The timeout value is the forward delay parameter
of the switches.
15 sec
Configuring the Hello Time
Enter the set spantree hello command to change the Hello time for a VLAN or for an MISTP instance.
The possible range for interval is from 1–10 seconds.
To configure the spanning tree bridge Hello time for a VLAN or an MISTP instance, perform this task
in privileged mode:
Task
Command
Step 1
Configure the Hello time for a VLAN or
MISTP instance.
set spantree hello interval [vlan] mistp-instance
[instances]
Step 2
Verify the configuration.
show spantree [vlan | mistp-instance instances]
This example shows how to configure the spanning tree Hello time for VLAN 100 to 7 seconds:
Console> (enable) set spantree hello 7 100
Spantree 100 hello time set to 7 seconds.
Console> (enable)
This example shows how to set the spantree Hello time for an instance to 3 seconds:
Console> (enable) set spantree hello 3 mistp-instance 1
Spantree 1 hello time set to 3 seconds.
Console> (enable)
7-44
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Configuring Spanning Tree Timers
Configuring the Forward Delay Time
Enter the set spantree fwddelay command to configure the spanning tree forward delay time for a
VLAN. The possible range for delay is from 4–30 seconds.
To configure the spanning tree forward delay time for a VLAN, perform this task in privileged mode:
Task
Command
Step 1
Configure the forward delay time for a VLAN or set spantree fwddelay delay [vlan]
MISTP instance.
mistp-instance [instances]
Step 2
Verify the configuration.
show spantree [mod/port] mistp-instance
[instances] [active]
This example shows how to configure the spanning tree forward delay time for VLAN 100 to 21 seconds:
Console> (enable) set spantree fwddelay 21 100
Spantree 100 forward delay set to 21 seconds.
Console> (enable)
This example shows how to set the bridge forward delay for an instance to 16 seconds:
Console> (enable) set spantree fwddelay 16 mistp-instance 1
Instance 1 forward delay set to 16 seconds.
Console> (enable)
Configuring the Maximum Aging Time
Enter the set spantree maxage command to change the spanning tree maximum aging time for a VLAN
or an instance. The possible range for agingtime is from 6–40 seconds.
To configure the spanning tree maximum aging time for a VLAN or an instance, perform this task in
privileged mode:
Task
Command
Step 1
Configure the maximum aging time for a VLAN
or MISTP instance.
set spantree maxage agingtime [vlans]
mistp-instance instances
Step 2
Verify the configuration.
show spantree [mod/port] mistp-instance
[instances] [active]
This example shows how to configure the spanning tree maximum aging time for VLAN 100 to
36 seconds:
Console> (enable) set spantree maxage 36 100
Spantree 100 max aging time set to 36 seconds.
Console> (enable)
This example shows how to set the maximum aging time for an instance to 25 seconds:
Console> (enable) set spantree maxage 25 mistp-instance 1
Instance 1 max aging time set to 25 seconds.
Console> (enable)
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-45
Chapter 7
Configuring Spanning Tree
Configuring MST
Configuring MST
The following sections describe how to configure MST:
Enabling MST
To enable and configure MST on the switch, perform this task in privileged mode:
Task
Command
Step 1
Begin in PVST+ mode.
set spantree mode mst [mistp | pvst+ | mistp-pvst+ | mst]
Step 2
Display the STP ports.
show spantree active
Step 3
Configure the MST region.
set spantree mst config {[name name] | [revision number]
[commit | rollback | force]}
Step 4
Verify your configuration.
show spantree mst config
Step 5
Map VLANs to the MST instance.
set spantree mst instance vlan vlan
Step 6
Commit the new region mapping.
set spantree mst config commit
Step 7
Enable MST.
set spantree mode mst [mistp | pvst+ | mistp-pvst+ | mst]
Step 8
Verify your MST configuration.
show spantree mst config
Step 9
Verify your MST instance configuration. show spantree mst instance
Step 10
Verify your MST module and port
configuration.
show spantree mst mod/port
These examples show how to enable MST:
Console>
Console>
Spantree
Console>
VLAN 1
Spanning
Spanning
Spanning
(enable)
(enable) set spantree mode pvst
mode set to PVST+.
(enable) show spantree active
tree mode
tree type
tree enabled
PVST+
ieee
Designated Root
00-50-3e-66-d0-00
Designated Root Priority
24576
Designated Root Cost
104
Designated Root Port
6/1
Root Max Age
20 sec
Hello Time 2 sec
Forward Delay 15 sec
Bridge ID MAC ADDR
Bridge ID Priority
Bridge Max Age 20 sec
Port
Channel_id
--------------------------------6/1
6/2
Console> (enable)
7-46
00-10-7b-bb-2f-00
32768
Hello Time 2 sec
Forward Delay 15 sec
Vlan Port-State
Cost
Prio Portfast
---- ------------- --------- ---- -------1
1
forwarding
blocking
4
4
32 disabled 0
32 disabled 0
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Configuring MST
Console> (enable) set spantree mst config name cisco revision 1
Edit Buffer modified. Use 'set spantree mst config commit' to apply the
changes.
Console> (enable) show spantree mst config
Current (NVRAM) MST Region Configuration:
Configuration Name:
Revision:0
Instance VLANs
-------- -------------------------------------------------------------IST
1-4094
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
=======================================================================
NEW MST Region Configuration (Not committed yet)
Configuration Name:cisco
Revision:1
Instance VLANs
-------- -------------------------------------------------------------IST
1-4094
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
=======================================================================
Edit buffer is locked by:Console (pid 142)
Console> (enable)
Console> (enable) set spantree mst 1 vlan 2-10
Edit Buffer modified. Use 'set spantree mst config commit' to apply the
changes.
Console> (enable) set spantree mst 1 vlan 2-20
Edit Buffer modified. Use 'set spantree mst config commit' to apply the
changes.
Console> (enable) set spantree mst 2 vlan 21-30
Edit Buffer modified. Use 'set spantree mst config commit' to apply the
changes.
Console> (enable) set spantree mst 3 vlan 31-40
Edit Buffer modified. Use 'set spantree mst config commit' to apply the
changes.
Console> (enable) set spantree mst 4 vlan 41-50
Edit Buffer modified. Use 'set spantree mst config commit' to apply the
changes.
Console> (enable)
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-47
Chapter 7
Configuring Spanning Tree
Configuring MST
Console> (enable) show spantree mst config
Current (NVRAM) MST Region Configuration:
Configuration Name:
Revision:0
Instance VLANs
-------- -------------------------------------------------------------IST
1-4094
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
=======================================================================
NEW MST Region Configuration (Not committed yet)
Configuration Name:cisco
Revision:1
Instance VLANs
-------- -------------------------------------------------------------IST
1,51-4094
1
2-20
2
21-30
3
31-40
4
41-50
5
6
7
8
9
10
11
12
13
14
15
=======================================================================
Edit buffer is locked by:Console (pid 142)
Console> (enable)
Console> (enable) set spantree mst config commit
Console> (enable)
Console> (enable) show spantree mst config
Current (NVRAM) MST Region Configuration:
Configuration Name:cisco
Revision:1
Instance VLANs
-------- -------------------------------------------------------------IST
1,51-4094
1
2-20
2
21-30
3
31-40
4
41-50
5
6
7
8
9
10
11
-
7-48
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Configuring MST
12
13
14
15
=======================================================================
Console> (enable)
Console> (enable) set spantree mode mst
PVST+ database cleaned up.
Spantree mode set to MST.
Console> (enable)
Console> (enable)
Console> (enable) show spantree mst 0
Spanning tree mode
MST
Instance
0
VLANs Mapped:
1,51-4094
Designated Root
Designated Root Priority
Designated Root Cost
Designated Root Port
Root Max Age
20 sec
00-50-3e-66-d0-00
24576 (root priority:24576, sys ID ext:0)
20100
6/1
Hello Time 2
sec
Forward Delay 15 sec
IST Master ID MAC ADDR
IST Master ID Priority
IST Master Path Cost
00-10-7b-bb-2f-00
32768
0
Remaining Hops 20
Bridge ID MAC ADDR
Bridge ID Priority
0)
Bridge Max Age 20 sec
Hops 20
00-10-7b-bb-2f-00
32768 (bridge priority:32768, sys ID ext:
Hello Time 2
Port
State
------------------------ -------------------------------6/1
forwarding
Boundary(PVST)
6/2
blocking
Boundary(PVST)
Console> (enable) show spantree mst 1
Spanning tree mode
MST
Instance
1
VLANs Mapped:
2-20
sec
Forward Delay 15 sec
Max
Role Cost
Prio Type
---- -------- ---ROOT
20000
32 P2P,
ALTR
20000
32 P2P,
Designated Root
Designated Root Priority
00-10-7b-bb-2f-00
32769 (root priority:32768, sys ID ext:1)
Designated Root Cost
Designated Root Port
0
1/0
Bridge ID MAC ADDR
Bridge ID Priority
1)
00-10-7b-bb-2f-00
32769 (bridge priority:32768, sys ID ext:
Port
------------------------------------------6/1
Boundary(PVST)
6/2
Boundary(PVST)
Console> (enable)
Console> (enable)
Remaining Hops 20
State
Role Cost
Prio Type
------------- ---- -------- ---forwarding
BDRY
20000
32 P2P,
blocking
BDRY
20000
32 P2P,
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-49
Chapter 7
Configuring Spanning Tree
Configuring MST
Console> (enable) show spantree mst 6/1
Edge Port:
No, (Configured) Default
Link Type:
P2P, (Configured) Auto
Port Guard:
Default
Boundary:
Yes (PVST)
Inst State
Role Cost
Prio VLANs
---- ------------- ---- --------- -------------------------------------0 forwarding
ROOT
20000
32 1
1 forwarding
BDRY
20000
32 2-20
2 forwarding
BDRY
20000
32 21-30
3 forwarding
BDRY
20000
32 31-40
4 forwarding
BDRY
20000
32 41-50
Console> (enable)
Console> (enable)
Console> (enable) show spantree mst config
Current (NVRAM) MST Region Configuration:
Configuration Name:cisco
Revision:1
Instance VLANs
-------- -------------------------------------------------------------IST
1,51-4094
1
2-20
2
21-30
3
31-40
4
41-50
5
6
7
8
9
10
11
12
13
14
15
=======================================================================
Console> (enable)
Configuring the MST Bridge ID Priority
You can set the bridge ID priority for an MST instance when the switch is in MST mode. The switch
combines the bridge priority value with the system ID extension (the ID of the MST instance) to create
the bridge ID priority. You can set 16 possible bridge priority values: 0, 4096, 8192, 12,288, 16,384,
20,480, 24,576, 28,672, 32,768, 36,864, 40,960, 45,056, 49,152, 53,248, 57,344, and 61,440.
To configure the bridge ID priority for an MST instance, perform this task in privileged mode:
Task
Command
Step 1
Configure the bridge ID priority for an
MST instance.
set spantree priority bridge_priority mst [instance]
Step 2
Verify the bridge ID priority.
show spantree mst [instance | mod/port]
The example shows how to configure the bridge ID priority for an MST instance:
Console> (enable) set spantree priority 8192 mst 3
MST Spantree 3 bridge priority set to 8192.
Console> (enable)
7-50
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Configuring MST
Console> (enable) show spantree mst 3
Spanning tree mode
MST
Instance
3
VLANs Mapped:
31-40
Designated
Designated
Designated
Designated
Root
Root Priority
Root Cost
Root Port
Bridge ID MAC ADDR
Bridge ID Priority
Port
------------------------------------------6/1
Boundary(PVST)
6/2
Boundary(PVST)
00-10-7b-bb-2f-00
8195 (root priority:8192, sys ID ext:3)
0
Remaining Hops 20
1/0
00-10-7b-bb-2f-00
8195 (bridge priority:8192, sys ID ext:3)
State
Role Cost
Prio Type
------------- ---- -------- ---forwarding
BDRY
20000
32 P2P,
blocking
BDRY
20000
32 P2P,
Configuring the MST Port Cost
You can configure the port cost of switch ports.The switch uses ports with lower port costs to forward
frames. Assign lower numbers to ports that are attached to faster media (such as full duplex) and higher
numbers to ports that are attached to slower media. The possible range is from 1–65,535. The default
differs for different media. The path cost is typically 1000 ÷ LAN speed in megabits per second.
To configure the port cost for a port, perform this task in privileged mode:
Task
Command
Step 1
Configure the MST port cost for a switch port.
set spantree portcost mod/port cost [mst]
Step 2
Verify the port cost setting.
show spantree mst [instance | mod/port]
This example shows how to configure the port cost on an MST instance and verify the configuration:
Console> (enable) set spantree portcost 6/1 10000 mst
Spantree port 6/1 path cost set to 10000.
Console> (enable)
Console> (enable) show spantree mst 6/1
Edge Port:
No, (Configured) Default
Link Type:
P2P, (Configured) Auto
Port Guard:
Default
Boundary:
Yes (PVST)
Inst State
Role Cost
Prio
---- ------------- ---- --------- -------------------------------------0 forwarding
ROOT
10000
32
1 forwarding
BDRY
10000
32
2 forwarding
BDRY
10000
32
3 forwarding
BDRY
10000
32
4 forwarding
BDRY
10000
32
Console> (enable)
VLANs
1
2-20
21-30
31-40
41-50
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-51
Chapter 7
Configuring Spanning Tree
Configuring MST
Configuring the MST Port Priority
You can configure the port priority of ports. The port with the lowest priority value forwards frames for
all VLANs. The possible port priority value is from 0–63; the default is 32. If all ports have the same
priority value, the port with the lowest port number forwards frames.
To configure the port priority for a port, perform this task in privileged mode:
Task
Command
Step 1
Configure the MST port priority for a port.
set spantree portpri mod/port priority [mst]
Step 2
Verify the port priority setting.
show spantree mst [instance | mod/port]
This example shows how to configure the port priority and verify the configuration:
Console> (enable) set spantree portpri 6/1 30 mst
Bridge port 6/1 port priority set to 30.
Console> (enable)
Console> (enable) show spantree mst 6/1
Edge Port:
No, (Configured) Default
Link Type:
P2P, (Configured) Auto
Port Guard:
Default
Boundary:
Yes (PVST)
Inst State
Role Cost
Prio
---- ------------- ---- --------- -------------------------------------0 forwarding
ROOT
10000
30
1 forwarding
BDRY
10000
30
2 forwarding
BDRY
10000
30
3 forwarding
BDRY
10000
30
4 forwarding
BDRY
10000
30
Console> (enable)
VLANs
1
2-20
21-30
31-40
41-50
Configuring the MST Port Instance Cost
You can configure the port instance cost for an instance of MST. Ports with a lower instance cost are
more likely to be chosen to forward frames. You should assign lower numbers to ports that are attached
to faster media (such as full duplex) and higher numbers to ports that are attached to slower media. The
default cost differs for different media. The possible value for port instance cost is from 1–268,435,456.
To configure the port instance cost for a port, perform this task in privileged mode:
Task
Command
Step 1
Configure the MST port instance cost on a port. set spantree portinstancecost mod/port [cost cost]
mst [instances]
Step 2
Verify the path cost for the instances on a port. show spantree portinstancecost mod/port
This example shows how to configure the MST port instance cost on a port:
Console>
Port 6/1
Port 6/1
Console>
7-52
(enable) set spantree portinstancecost 6/1 cost 5000 mst 4
MST Instances 0-3,5-15 have path cost 10000.
MST Instances 4 have path cost 5000.
(enable)
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Configuring MST
Console> (enable) show spantree mst 4
Spanning tree mode
MST
Instance
4
VLANs Mapped:
41-50
Designated Root
Designated Root Priority
00-10-7b-bb-2f-00
32772 (root priority:32768, sys ID ext:4)
Designated Root Cost
Designated Root Port
0
1/0
Bridge ID MAC ADDR
Bridge ID Priority
4)
00-10-7b-bb-2f-00
32772 (bridge priority:32768, sys ID ext:
Port
-----------------------6/1
Boundary(PVST)
6/2
Boundary(PVST)
Console> (enable)
Remaining Hops 20
State
Role Cost
Prio Type
------------- ---- -------- ----------------forwarding
BDRY
5000
30 P2P,
blocking
BDRY
20000
32 P2P,
Configuring the MST Port Instance Priority
You can set a port priority for an instance of MST. The port with the lowest priority for a specific MST
instance forwards frames for that instance. The port instance range is from 0–63. If all ports have the
same priority for an MST instance, the port with the lowest port number forwards frames for that
instance.
To configure the port instance priority on an MST instance, perform this task in privileged mode:
Task
Command
Step 1
Configure the port instance priority on an MST
instance.
set spantree portinstancepri mod/port
priority mst [instance]
Step 2
Verify the port instance priority setting.
show spantree mst [instance | mod/port]
This example shows how to configure the port instance priority on an MST instance and verify the
configuration:
Console> (enable) set spantree portinstancepri 6/1 20 mst 2
Port 6/1 MST Instances 2 using portpri 20.
Port 6/1 MST Instances 0-1,3-15 using portpri 30.
Console> (enable)
Console> (enable)
Console> (enable) show spantree mst 2
Spanning tree mode
MST
Instance
2
VLANs Mapped:
21-30
Designated Root
Designated Root Priority
00-10-7b-bb-2f-00
32770 (root priority:32768, sys ID ext:2)
Designated Root Cost
Designated Root Port
0
1/0
Remaining Hops 20
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-53
Chapter 7
Configuring Spanning Tree
Configuring MST
Bridge ID MAC ADDR
Bridge ID Priority
2)
Port
-----------------------6/1
Boundary(PVST)
6/2
Boundary(PVST)
Console> (enable)
00-10-7b-bb-2f-00
32770 (bridge priority:32768, sys ID ext:
State
Role Cost
Prio Type
------------- ---- -------- ----------------------forwarding
BDRY
10000
20 P2P,
blocking
BDRY
20000
32 P2P,
Mapping and Unmapping VLANs to an MST Instance
Note
See Chapter 10, “Configuring VLANs” for details on using VLANs.
By default, all VLANS are mapped to IST (instance 0). For an MST instance (MSTI) 1 through 15 to be
active, you must map at least one VLAN to that MSTI. IST will always be active whether VLANs are
mapped to IST or not. There are no VLAN mapping conflicts because of separate regions in MST.
Follow these guidelines for mapping and unmapping VLANS to an MST instance:
•
You can only map Ethernet VLANs to MST instances.
•
At least one VLAN in the instance must have an active port in order for MST to be active.
•
You can map as many Ethernet VLANs as you wish to an MST instance.
•
You cannot map a VLAN to more than one MST instance.
•
The Hello Time, Maximum Age timer, and Forward Delay timer set for mode and all spanning trees
are used globally by MST.
To map a VLAN to an MST instance, perform this task in privileged mode:
Task
Command
Step 1
Map a VLAN to an MST instance.
set spantree mst instance vlan vlan
Step 2
Make the new region mapping effective.
set spantree mst config commit
Step 3
Verify that the VLAN is mapped.
show spantree mst [instance] [active] mod/port
This example shows how to map a VLAN to MST instance 1 and verify the mapping:
Console> (enable) show spantree mst config
Current (NVRAM) MST Region Configuration:
Configuration Name:cisco
Revision:1
Instance VLANs
-------- -------------------------------------------------------------IST
1,51-4094
1
2-20
2
21-30
3
31-40
4
41-50
5
6
7
8
9
-
7-54
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Configuring MST
10
11
12
13
14
15
=======================================================================
Console> (enable)
Console> (enable) set spantree mst 14 vlan 900-999
Edit Buffer modified. Use 'set spantree mst config commit' to apply the
changes.
Console> (enable) show spantree mst config
Current (NVRAM) MST Region Configuration:
Configuration Name:cisco
Revision:1
Instance VLANs
-------- -------------------------------------------------------------IST
1,51-4094
1
2-20
2
21-30
3
31-40
4
41-50
5
6
7
8
9
10
11
12
13
14
15
=======================================================================
NEW MST Region Configuration (Not committed yet)
Configuration Name:cisco
Revision:2
Instance VLANs
-------- -------------------------------------------------------------IST
1,51-899,1000-4094
1
2-20
2
21-30
3
31-40
4
41-50
5
6
7
8
9
10
11
12
13
14
900-999
15
=======================================================================
Edit buffer is locked by:Console (pid 142)
Console> (enable)
Console> (enable) clear spantree mst 14 vlan 900-998
Edit Buffer modified. Use 'set spantree mst config commit' to apply the
changes.
Console> (enable)
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-55
Chapter 7
Configuring Spanning Tree
Configuring MST
Console> (enable) show spantree mst config
Current (NVRAM) MST Region Configuration:
Configuration Name:cisco
Revision:1
Instance VLANs
-------- -------------------------------------------------------------IST
1,51-4094
1
2-20
2
21-30
3
31-40
4
41-50
5
6
7
8
9
10
11
12
13
14
15
=======================================================================
NEW MST Region Configuration (Not committed yet)
Configuration Name:cisco
Revision:2
Instance VLANs
-------- -------------------------------------------------------------IST
1,51-998,1000-4094
1
2-20
2
21-30
3
31-40
4
41-50
5
6
7
8
9
10
11
12
13
14
999
15
=======================================================================
Edit buffer is locked by:Console (pid 142)
Console> (enable)
Console> (enable) set spantree mst config commit
Console> (enable)
Console> (enable) show spantree mst config
Current (NVRAM) MST Region Configuration:
Configuration Name:cisco
Revision:2
Instance VLANs
-------- -------------------------------------------------------------IST
1,51-998,1000-4094
1
2-20
2
21-30
3
31-40
4
41-50
5
6
7
8
9
10
11
-
7-56
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Configuring Spanning Tree BPDU Skewing
12
13
14
999
15
=======================================================================
Console> (enable)
Console> (enable) show spantree mst 3
Spanning tree mode
MST
Instance
3
VLANs Mapped:
31-40
Designated
Designated
Designated
Designated
Root
Root Priority
Root Cost
Root Port
Bridge ID MAC ADDR
Bridge ID Priority
Port
------------------------------------------6/1
Boundary(PVST)
6/2
Boundary(PVST)
Console> (enable)
00-10-7b-bb-2f-00
8195 (root priority:8192, sys ID ext:3)
0
Remaining Hops 20
1/0
00-10-7b-bb-2f-00
8195 (bridge priority:8192, sys ID ext:3)
State
Role Cost
Prio Type
------------- ---- -------- ---forwarding
BDRY
10000
30 P2P,
blocking
BDRY
20000
32 P2P,
Configuring Spanning Tree BPDU Skewing
Commands that support the spanning tree BPDU skewing feature perform these functions:
•
Allow you to enable or disable BPDU skewing. The default is disabled.
•
Modify the show spantree summary output to show if the skew detection is enabled and for which
VLANs or PVST+ or MISTP instances the skew was detected.
•
Provide a display of the VLAN or PVST+ or MISTP instance and the port that is affected by the
skew and include this information:
– The duration (in absolute time) of the last skew
– The duration (in absolute time) of the worst skew
– The date and time of the worst duration
To change how spanning tree performs BPDU skewing statistics gathering, enter the set spantree
bpdu-skewing command. The bpdu-skewing command is disabled by default.
To configure the BPDU skewing statistics gathering for a VLAN, perform this task in privileged mode:
Task
Command
Step 1
Configure BPDU skewing.
set spantree bpdu-skewing [enable | disable]
Step 2
Verify the configuration.
show spantree bpdu-skewing vlan [mod/port]
show spantree bpdu-skewing mistp-instance [instance] [mod/port]
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-57
Chapter 7
Configuring Spanning Tree
Configuring Spanning Tree BPDU Skewing
This example shows how to configure BPDU skewing and view the skewing statistics:
Console> (debug-eng) set spantree bpdu-skewing
Usage:set spantree bpdu-skewing <enable|disable>
Console> (debug-eng)
Console> (debug-eng)
Console> (debug-eng) set spantree bpdu-skewing enable
Spantree bpdu-skewing enabled on this switch.
Console> (debug-eng)
Console> (enable)
Console> (enable) show spantree bpdu-skewing 1
Bpdu skewing statistics for vlan 1
Port
Last Skew ms
Worst Skew ms
Worst Skew Time
------ ------------- ------------- ------------------------8/2
5869
108370 Tue Nov 21 2000, 06:25:59
8/4
4050
113198 Tue Nov 21 2000, 06:26:04
8/6
113363
113363 Tue Nov 21 2000, 06:26:05
8/8
4111
113441 Tue Nov 21 2000, 06:26:05
8/10
113522
113522 Tue Nov 21 2000, 06:26:05
8/12
4111
113600 Tue Nov 21 2000, 06:26:05
8/14
113678
113678 Tue Nov 21 2000, 06:26:05
8/16
4111
113755 Tue Nov 21 2000, 06:26:05
8/18
113833
113833 Tue Nov 21 2000, 06:26:05
8/20
4111
113913 Tue Nov 21 2000, 06:26:05
8/22
113917
113917 Tue Nov 21 2000, 06:26:05
8/24
4110
113922 Tue Nov 21 2000, 06:26:05
8/26
113926
113926 Tue Nov 21 2000, 06:26:05
8/28
4111
113931 Tue Nov 21 2000, 06:26:05
Console> (enable)
This example shows how to configure BPDU skewing for VLAN 1 on module 8, port 4 and view the
skewing statistics:
Console> (enable) show spantree bpdu-skewing 1 8/4
Bpdu skewing statistics for vlan 1
Port
Last Skew ms
Worst Skew ms
Worst Skew Time
------ ------------- ------------- ------------------------8/4
5869
108370 Tue Nov 21 2000, 06:25:59
You will receive a similar output when MISTP is running.
The show spantree summary command shows if BPDU skew detection is enabled and also lists the
VLANs or instances that are affected in the skew. This example shows the output of the show spantree
summary command:
Console> (enable) show spantree summary
Root switch for vlans: 1
BPDU skewing detection enabled for the bridge
BPDU skewed for vlans: 1
Portfast bpdu-guard disabled for bridge.
Portfast bpdu-filter disabled for bridge.
Uplinkfast disabled for bridge.
Backbonefast disabled for bridge.
Summary of connected spanning tree ports by vlan
VLAN Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ---------1
6
4
2
0
12
7-58
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7
Configuring Spanning Tree
Configuring Spanning Tree BPDU Skewing
Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ---------Total
6
4
2
0
12
Console> (enable)
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
7-59
Chapter 7
Configuring Spanning Tree
Configuring Spanning Tree BPDU Skewing
7-60
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01