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Understanding How MST Works
To the spanning tree protocol running in the SST region, an MST region appears as a single SST or
pseudobridge. Pseudobridges operate as follows:
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Common Spanning Tree
802.1Q specifies a single spanning tree for all the VLANs called CST. In a Catalyst 4500 series switch
running PVST+, the VLAN 1 spanning tree corresponds to CST. In a Catalyst -4500 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
MST configuration has three parts as follows:
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Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
7-18
The same values for root identifiers and root path costs are sent in all BPDUs of all the pseudobridge
ports. Pseudobridges differ from a single SST bridge as follows:
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The pseudobridge BPDUs have different bridge identifiers. This difference does not affect STP
operation in the neighboring SST regions because the root identifier and root cost are the same.
BPDUs sent from the pseudobridge ports may have significantly different message ages.
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Because the message age increases by 1 second for each hop, the difference in the message age
is in the order of seconds.
Data traffic from one port of a pseudobridge (a port at the edge of a region) to another port follows
a path entirely contained within the pseudobridge or MST region.
Data traffic belonging to different VLANs may follow different paths within the MST regions
established by MST.
Loop prevention is achieved by either of the following:
Blocking the appropriate pseudobridge ports by allowing one forwarding port on the boundary
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and blocking all other ports.
Setting the CST partitions to block the ports of the SST regions.
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A pseudo bridge differs from a single SST bridge because the BPDUs sent from the pseudobridge's
ports have different bridge identifiers. The root identifier and root cost are the same for both bridges.
Name—A 32-character string (null padded and null terminated) identifying the MST region.
Revision number—An unsigned 16-bit number that increments each time a change is made to the
configuration.
You must set and update the revision number manually, because it does not auto-increment
Note
each time you commit the MST configuration.
Chapter 7
Configuring Spanning Tree
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Release 8.1
78-15486-01
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- Configuration Guide
- Catalyst 2980G Switch
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