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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 12 Configuring RSTP and MSTP
Configuring RSTP and MSTP Featur es
configuration command. T o return to the default revision number, use the no revision MST conf igurati on
comman d. To re-enable PV ST, use the no spanning-tree mode or the spanning-tree mode pvst glob al
configurati on comm a nd.
This example shows how to enter M ST configurati on mode, ma p VLANs 10 to 20 to MST instanc e 1,
name the region region1, set the configuration revision to 1, display the pending configuration, apply the
chan ges, an d r etu rn to g loba l c onfigu rat ion m od e:
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 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
configurati on comm a nd to m odif y th e sw itch prio ri ty f rom th e default value (32768) t o a sign ifican tly
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 syst em ID supp ort, the swi tch se ts i ts own prior ity f or the spec ified inst ance to 24 576 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
o wn prior ity to 4 096 less tha n the lo west swit ch priori ty. (4 096 is the v alu e of the least -signif ic ant bit of
a 4-bit switch priority va lue as shown in Table 11-1 on pa ge 11-4.)
Note Ca talyst 295 0 sw itc hes ru nnin g so ftwa re e arl ier tha n Rele as e 12. 1( 9)EA 1 d o not s uppo rt t h e extend ed
system ID. Catalyst 2950 switches running software earlier than Release 12.1(9)EA1 do not support the
MSTP.
Note 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.