Configuring The Root Switch - Cisco Catalyst 3550 series Software Configuration Manual

Configuring MSTP Features
Command
Step 8 spanning-tree mode mst
Step 9 end
Step 10
show running-config
Step 11
copy running-config startup-config
To return to the default MST region configuration, use the no spanning-tree mst configuration global
configuration command. To return to the default VLAN-to-instance map, use the no instance instance-id
[vlan vlan-range] MST configuration command. To return to the default name, use the no name MST
configuration command. To return to the default revision number, use the no revision MST configuration
command. To re-enable PVST+, use the no spanning-tree mode or the spanning-tree mode pvst global
configuration command.
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:
Switch(config)# spanning-tree mst configuration
Switch(config-mst)# instance 1 vlan 10-20
Switch(config-mst)# name region1
Switch(config-mst)# revision 1
Switch(config-mst)# show pending
Pending MST configuration
Name
Revision
Instance
--------
0
1
-------------------------------
Switch(config-mst)# exit
Switch(config)#

Configuring the Root Switch

The switch maintains a spanning-tree instance for the group of VLANs mapped to it. A bridge ID,
consisting of the switch priority and the switch MAC address, is associated with each instance. The
switch with the lowest bridge ID becomes the root switch for the group of VLANs.
To configure a switch to become the root, 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 switch for the specified spanning-tree instance. When
you enter this command, the switch checks the switch priorities of the root switches. Because of the
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 for the specified spanning-tree instance.
Catalyst 3550 Multilayer Switch Software Configuration Guide
17-14
Purpose
Enable MSTP. RSTP is also enabled.
Caution
You cannot run both MSTP and PVST+ or both MSTP and rapid PVST+
at the same time.
Return to privileged EXEC mode.
Verify your entries.
(Optional) Save your entries in the configuration file.
[region1]
1
Vlans Mapped
---------------------
1-9,21-4094
10-20
Changing spanning-tree modes can disrupt traffic because all
spanning-tree instances are stopped for the previous mode and
restarted in the new mode.
Chapter 17 Configuring MSTP
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