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15-13
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
78-11380-10
Chapter 15 Configuring MSTP
Configuring MSTP Features
of the MST regions must contain the CST root, and all of the other MST regions must have a better
path to the root contained within the MST cloud than a path through the PVST+ or rapid-PVST+
cloud. You might have to manually configure the switches in the clouds.
• Partitioning the network into a large number of regions is not recommended. However, if this
situation is unavoidable, we recommend that you partition the switched LAN into smaller LANs
interconnected by routers or non-Layer 2 devices.
• For configuration guidelines about UplinkFast, BackboneFast, and cross-stack UplinkFast, see the
“Optional Spanning-Tree Configuration Guidelines” section on page 16-13.
Specifying the MST Region Configuration and Enabling MSTP
For two or more switches to be in the same MST region, they must have the same VLAN-to-instance
mapping, the same configuration revision number, and the same name.
A region can have one member or multiple members with the same MST configuration; each member
must be capable of processing RSTP BPDUs. There is no limit to the number of MST regions in a
network, but each region can support up to 16 spanning-tree instances. You can assign a VLAN to only
one spanning-tree instance at a time.
Beginning in privileged EXEC mode, follow these steps to specify the MST region configuration and
enable MSTP. This procedure is required.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
spanning-tree mst configuration Enter MST configuration mode.
Step 3
instance instance-id vlan vlan-range Map VLANs to an MST instance.
• For instance-id, you can specify a single instance, a range of
instances separated by a hyphen, or a series of instances separated by
a comma. The range is 1 to 15.
• For vlan vlan-range, the range is 1 to 1005 when the SI is installed
and 1 to 4094 when the EI is installed.
When you map VLANs to an MST instance, the mapping is incremental,
and the VLANs specified in the command are added to or removed from
the VLANs that were previously mapped.
To specify a VLAN range, use a hyphen; for example, instance 1 vlan
1-63 maps VLANs 1 through 63 to MST instance 1.
To specify a VLAN series, use a comma; for example, instance 1 vlan 10,
20, 30 maps VLANs 10, 20, and 30 to MST instance 1.
Step 4
name name Specify the configuration name. The name string has a maximum length
of 32 characters and is case sensitive.
Step 5
revision version Specify the configuration revision number. The range is 0 to 65535.
Step 6
show pending Verify your configuration by displaying the pending configuration.
Step 7
exit Apply all changes, and return to global configuration mode.