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MST General Overview Using 802.1s Multiple Spanning Tree
page 6-8 OmniSwitch 6600 Family Network Configuration Guide April 2006
What is a Multiple Spanning Tree Region
A Multiple Spanning Tree region represents a group of 802.1s switches. An MST region appears as a
single, flat mode instance to switches outside the region. A switch can belong to only one region at a time.
The region a switch belongs to is identified by the following configurable attributes, as defined by the
IEEE 802.1s standard:
• Region name–An alphanumeric string up to 32 characters.
• Region revision level–A numerical value between 0 and 65535.
• VLAN to MSTI table–Generated when VLANs are associated with MSTIs. Identifies the VLAN to
MSTI mapping for the switch.
Switches that share the same values for the configuration attributes described above belong to the same
region. For example, in the diagram below:
• Switches A, B, and C all belong to the same region because they all are configured with the same
region name, revision level, and have the same VLANs mapped to the same MSTI.
• The CST for the entire network sees Switches A, B, and C as one virtual bridge that is running a single
Spanning Tree instance. As a result, CST blocks the path between Switch C and Switch E instead of
blocking a path between the MST region switches to avoid a network loop.
• The paths between Switch A and Switch C and the redundant path between Switch B and Switch C
were blocked as a result of the Internal Spanning Tree (IST) computations for the MST Region. See
“What is the Internal Spanning Tree (IST) Instance” on page 6-9 for more information.
In addition to the attributes described above, the MST maximum hops parameter defines the number of
bridges authorized to propagate MST BPDU information. In essence, this value defines the size of the
region in that once the maximum number of hops is reached, the BPDU is discarded. The maximum
MST Region
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Switch C
Switch B
Switch A
Switch E
Switch D
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SST Switches (STP or RSTP)
CST
IST