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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 14 Configuring MSTP
Configuring MSTP Features
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 [region1]
Revision 1
Instance Vlans Mapped
-------- ---------------------
0 1-9,21-4094
1 10-20
-------------------------------
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 switch ID,
consisting of the switch priority and the switch MAC address, is associated with each instance. The
switch with the lowest switch 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.
If any root switch 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 13-1 on page 13-4.)
Note Catalyst 2950 switches running software earlier than Cisco IOS Release 12.1(9)EA1 do not support the
extended system
ID. Catalyst 2950 switches running software earlier than Cisco IOS Release
12.1(9)EA1 do not support the MSTP.
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 switch. 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 switch for each spanning-tree instance should be a backbone or distribution switch. Do not
configure an access switch as the spanning-tree primary root.