802.1s Multiple Spanning Tree Protocol (MSTP)
How Separate Instances Affect MSTP Operation. Assigning different groups of VLANs to different instances ensures that those VLAN groups use independent forwarding paths. For example, in figure
Path through IST Instance to Other Regions
Region “X”
Switch 1
IST Root
VLAN Memberships:
• IST Instance: VLANs 1, 2
• MSTI “A”: 4, 5
• MSTI “B”: 7, 9
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| link for MSTI “B”. |
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| link for MSTI “A”. |
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| Switch 2 |
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| Switch 3 | ||||||||||||||||||
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| MSTI “A” Root |
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| MSTI “B” Root | ||||||||||||||||||||
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VLAN Memberships: |
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| VLAN Memberships: | ||||||||||||||||||||||||||
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• IST Instance: VLANs 1, 2 |
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| • IST Instance: VLANs 1, 2 | |||||||||||||||||||||||||||||||||||||||
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| link for IST instance. |
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• MSTI “A”: 4, 5 |
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| • MSTI “A”: 4, 5 | |||||||||||||||||||||||||||||||||||||||
• MSTI “B”: 7, 9 |
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| • MSTI “B”: 7, 9 | ||||||||||||||||||||||||||
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While allowing only one active path through a given instance, MSTP retains any redundant physical paths in the instance to serve as backups (blocked) paths in case the existing active path fails. Thus, if an active path in an instance fails, MSTP automatically activates (unblocks) an available backup to serve as the new active path through the instance for as long as the original active path is down. Note also that a given port may simultaneously operate in different states (forwarding or blocking) for different