Cisco 4500M User Manual

16-12
Software Configuration Guide—Release 12.2(25)EW
OL-6696-01
Chapter 16 Understanding and Configuring Multiple Spanning Trees
Configuring MST
Configuring MST Instance Port Parameters
To configure MST instance port parameters, perform this task:
This example shows how to configure MST instance port parameters:
Switch(config)# interface fastethernet 4/4
Switch(config-if)# spanning-tree mst 1 ?
cost Change the interface spanning tree path cost for an instance
port-priority Change the spanning tree port priority for an instance
Switch(config-if)# spanning-tree mst 1 cost 1234567
Switch(config-if)# spanning-tree mst 1 port-priority 240
Switch(config-if)# ^Z
Switch# show spanning-tree mst 1 interface fastethernet 4/4
FastEthernet4/4 of MST01 is backup blocking
Edge port:no (default) port guard :none (default)
Link type:point-to-point (auto) bpdu filter:disable (default)
Boundary :internal bpdu guard :disable (default)
Bpdus (MRecords) sent 125, received 1782
Instance Role Sts Cost Prio.Nbr Vlans mapped
-------- ---- --- --------- -------- -------------------------------
1 Back BLK 1234567 240.196 1-10
Switch#
Restarting Protocol Migration
RSTP and MST have built-in compatibility mechanisms that allow them to interact properly with other
regions or other versions of IEEE spanning-tree. For example, an RSTP bridge connected to a legacy
bridge can send 802.1D BPDUs on one of its ports. Similarly, when an MST bridge receives a legacy
BPDU or an MST BPDU associated with a different region, it is also to detect that a port is at the
boundary of a region.
Unfortunately, these mechanisms cannot always revert to the most efficient mode. For example, an RSTP
bridge designated for a legacy 802.1D will stay in 802.1D mode even after the legacy bridge has been
removed from the link. Similarly, an MST port still assumes that it is a boundary port when the bridge(s)
to which it is connected have joined the same region. To force a Catalyst 4000 family switch to
renegotiate with the neighbors (that is, to restart protocol migration), you must enter the clear
spanning-tree detected-protocols command, as follows:
Switch# clear spanning-tree detected-protocols fastethernet 4/4
Switch#
Command Purpose
Step 1
Switch(config-if)# spanning-tree mst x cost y
Configures the MST instance port cost.
Step 2
Switch(config-if)# spanning-tree mst x port-priority y
Configures the MST instance port priority.
Step 3
Switch(config-if)# Ctrl-Z
Exits configuration mode.
Step 4
Switch# show spanning-tree mst x interface y
Verifies the configuration.

Cisco 4500M Specifications

General

Brand	Cisco
Model	4500M
Category	Network Router
Language	English