Common Spanning Tree; Mst Instances; Mst Configuration Parameters - Cisco 6500 Series Software Configuration Manual

Understanding How IEEE 802.1s MST Works
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 this case, the topology changes are only propagated
in the instance to which the VLAN is mapped. The topology change stays local to the first MST region
and the 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.

Common Spanning Tree

CST (802.1Q) is a single spanning tree for all the VLANs. In a Catalyst 6000 family switch running
PVST+, the VLAN 1 spanning tree corresponds to CST. In a Catalyst 6500 series switch running MST,
IST (instance 0) corresponds to CST.

MST Instances

This release supports up to 16 instances; each spanning tree instance is identified by an instance ID that
ranges from 0 to 15. Instance 0 is mandatory and is always present. Instances 1 through 15 are optional.

MST Configuration Parameters

MST configuration includes these three parts:
•
•
•
You must configure each byte manually. You can use SNMP or the CLI to perform the configuration.
MST BPDUs contain the MST configuration ID and the checksum. An MST bridge accepts an MST
BPDU only if the MST BPDU configuration ID and the checksum match its own MST region
configuration ID and checksum. If one value is different, the MST BPDU is considered to be an
SST BPDU.
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Name—A 32-character string (null padded) identifying the MST region.
Revision number—An unsigned 16-bit number that identifies the revision of the current MST
configuration.
You must set the revision number when required as part of the MST configuration. The
Note
revision number is not incremented automatically each time you commit the MST
configuration.
MST configuration table—An array of 4096 bytes. Each byte, interpreted as an unsigned integer,
corresponds to a VLAN. The value is the instance number to which the VLAN is mapped. The first
byte that corresponds to VLAN 0 and the 4096th byte that corresponds to VLAN 4095 are unused
and always set to zero.
Chapter 15 Configuring STP and IEEE 802.1s MST
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