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18-29
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 18 Configuring STP and MST
MST Configuration Restrictions and Guidelines
• Do not locate the root for some or all of the VLANs inside the PVST+ side of the MST switch
because when the MST switch at the boundary receives PVST+ BPDUs for all or some of the
VLANs on its designated ports, root guard sets the port to the blocking state.
When you connect a PVST+ switch to two different MST regions, the topology change from the PVST+
switch does not pass beyond the first MST region. In such a case, the topology changes are propagated
only in the instance to which the VLAN is mapped. The topology change stays local to the first MST
region, and the Cisco Access Manager (CAM) entries in the other region are not flushed. To make the
topology change visible throughout other MST regions, you can map that VLAN to IST or connect the
PVST+ switch to the two regions through access links.
MST Configuration Restrictions and Guidelines
Follow these restrictions and guidelines to avoid configuration problems:
• Do not disable spanning tree on any VLAN in any of the PVST bridges.
• Do no use PVST bridges as the root of CST.
• Do not connect switches with access links because access links may partition a VLAN.
• Ensure that all PVST root bridges have lower, (numerically higher) priority than the CST root
bridge.
• Ensure that trunks carry all of the VLANs mapped to an instance or do not carry any VLANs at all
for this instance.
• Complete any MST configuration that incorporates a large number of either existing or new logical
VLAN ports during a maintenance window because the complete MST database gets reinitialized
for any incremental change (such as adding new VLANs to instances or moving VLANs across
instances).
Configuring MST
The following sections describe how to configure MST:
• Enabling MST, page 18-29
• Configuring MST Instance Parameters, page 18-31
• Configuring MST Instance Port Parameters, page 18-32
• Restarting Protocol Migration, page 18-33
• Displaying MST Configurations, page 18-33
Enabling MST
To enable and configure MST on a Catalyst 4500, perform this task:
Command Purpose
Step 1
Switch(config)# spanning-tree mode mst
Enters MST mode.