Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 264

Configuring MST Chapter 17 Configuring STP and MST This example shows how to enter MST configuration mode, map VLANs 10 to 20 to MST instance 1, name the region region1, set the configuration revision to 1, display the pending configuration, apply the changes, and return to global configuration mode: Router(config)# spanning-tree mst configuration Router(config-mst)# instance 1 vlan 10-20 Router(config-mst)# name region1 Router(config-mst)# revision 1 Router(config-mst)# show pending Pending MST configuration Name [region1] Revision 1 Instances configured 2 Instance Vlans Mapped 0 1-9,21-4094 1 10-20 Router(config-mst)# exit Router(config)# Configuring the Root Bridge The switch maintains a spanning tree instance for the group of VLANs mapped to it. A switch ID, consisting of the switch priority and the switch MAC address, is associated with each instance. For a group of VLANs, the switch with the lowest switch ID becomes the root bridge. To configure a switch to become the root bridge, use the spanning-tree mst instance_id root global configuration command to modify the switch priority from the default value (32768) to a significantly lower value so that the switch becomes the root bridge for the specified spanning tree instance. When you enter this command, the switch checks the switch priorities of the root bridges. Because of extended system ID support, the switch sets its own priority for the specified instance to 24576 if this value will cause this switch to become the root bridge for the specified spanning tree instance. If any root bridge for the specified instance has a switch priority lower than 24576, the switch sets its own priority to 4096 less than the lowest switch priority. (4096 is the value of the least-significant bit of a 4-bit switch priority value as shown in Table 17-1 on page 17-3.) If your network consists of switches that both do and do not support the extended system ID, it is unlikely that the switch with the extended system ID support will become the root bridge. The extended system ID increases the switch priority value every time the VLAN number is greater than the priority of the connected switches running older software. The root bridge for each spanning tree instance should be a backbone or distribution switch. Do not configure an access switch as the spanning tree primary root bridge. Use the diameter keyword, which is available only for MST instance 0, to specify the Layer 2 network diameter (that is, the maximum number of Layer 2 hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time. Note With the switch configured as the root bridge, do not manually configure the hello time, forward-delay time, and maximum-age time with the spanning-tree mst hello-time, spanning-tree mst forward-time, and spanning-tree mst max-age global configuration commands. 17-40 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY OL-11439-03