Chapter 7: Spanning Tree Protocol (STP) 117
Section 7-2
By default, all Catalyst switches run PVST+ STP for one instance of STP on each
VLAN. To configure other STP modes, rapid-pvst (802.1w per vlan with IEEE com-
patibility) or mst must be explicitly enabled.
3. (MST only) Activate an MST instance:
a. Enter MST Configuration mode:
spanning-tree mst configuration
b. Identify the MST region:
name name
revision revision-number
The MST region is identified by name (a text string up to 32 characters). If no
name is given, no region name is used. You can use a region revision number to
indicate the number of times the region configuration has changed. The revision-
number (0 to 65535, default 1) must be explicitly set and is not automatically
incremented with region changes.
c. Map one or more VLANs to the instance:
instance instance-id vlan vlan-range
A vlan number (1 to 1005, 1025 to 4094) is mapped to the MST instance (0 to
15). This mapping is held in the MST region buffer until the changes are commit-
ted.
d. Exit MST configuration mode:
end
When you end this mode, you will be returned to privileged EXEC mode. The
changes to the configuration will be immediate, but you must save the configura-
tion to NVRAM to make them permanent.
4. (Optional) Placement of the root bridge switch.
Note The root bridge (and secondary root bridges) should be placed near the “center” of
the network so that an optimum spanning-tree topology is computed. Typically, the root is
located in the core or distribution layers of the network. If you choose not to manually
configure the root placement, the switch with the lowest BID wins the root election. This
almost always produces a spanning-tree topology that is inefficient.
(global) spanning-tree vlan vlan root {primary | secondary} [diameter net-
diameter [hello-time hello-time]]
The switch becomes the primary root bridge for the VLANs (a list of VLAN num-
bers 1 to 1005 and 1025 to 4094) or STP instances (1 to 16) specified (VLAN 1 if
unspecified). The bridge priority value is modified as follows: If it is more than 8192,
it is set to 8192; if it is already less than 8192, it is set to a value less than the current
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