multiple spanning trees shyam kaluve ( [email protected] ) ieee 802.1s 05/21/2001 st. louis interim
TRANSCRIPT
05/21/2001 IEEE 802.1s - St. Louis Interim - Shyam Kaluve 2
Common Spanning Tree (Now)
Root
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Common Spanning Tree (802.1s)
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Typical Deployment
• A standalone 802.1s network
• Old SST bridges in access layer
• CST root in MST backbone
• MSTs must not be affected by CST
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MST Super Bridge
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Single Bridge vs Super Bridge
• Switch fabric - MST instances
• Fabric structure is internal to the bridge - MST structure must be internal
• Fabric operation independent of CST- So should be MST operation
• Sync/ReRoot are atomic- Should propagate a cut in all MSTs
• Bridge ID,Message Age are different
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R
Single Bridge vs Super Bridge
Path Costs are not accurate
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R
Single Bridge vs Super Bridge
A B C
Designated Bridge ID does not matter
YZ X
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MST Region
Transitive closure of MST bridges• Either this MST Bridge is the only
member of the MST Region• Or this MST bridge is interconnected
by an internal LAN to one or more MST bridges having the same Region Config
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Internal LAN
LAN L is internal to a bridge B if the designated bridge D for L also has the same Region Configuration as B
Thanks to Mick
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MST Region
R
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ISTP
Holds Super Bridge together
• Distributes CST info
• Conveys MST Region Config
• Identifies boundaries
• Conveys Sync/ReRoot of Super Bridge
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Super Sync
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Super Sync
• All MST ports are forcefully blocked
• Till agreement on IST Master is reached
• Request for Super Sync is propagated to downstream bridges in IST
• May result in partial Super Sync that reverses direction
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Partial Super Sync
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MSTP
Current 7 component priority vector
• Alters M-trees unnecessarily
• Results in instability every time CST priority changes
• Convergence follows CST convergence• Either requires more space or propagates
inconsistent CST info
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MSTP
• Regular 4 component priority vector is sufficient
• Operates the Super Bridge’s internal switch fabric
• Totally unaffected by CST changes in a single 802.1s cloud
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Boundary Ports
• IST port state overrides M-ports
• All M-ports are edge ports
• IST Root/Designated MST Designated
• IST Alternate MST Overridden
• IST Backup MST Designated/Alternate
• IST+MST CST
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Rapid Transition
• If IST port is root port and becomes forwarding
At the boundary
• Or if IST port is Designated and gets an agreement
- then set agreement for all M-ports
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ISTP
• {C-Root, C-Cost, C-Bridge, C-Port, I-Master, I-Cost, I-Bridge, I-Port}
• Include CST Designated Bridge and Port ID to make an 8 component vector
• Introduce IST Flags to indicate SuperSyncRequest
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Independence From CST
• MST instances can use totally different algorithm
• Separate hello-timer, forward-delay, max-age within the region (?)
• Change in Super Bridge’s root port does not necessarily cause a Super Sync
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Conclusion
• ISTP makes the Super Bridge– Identifies boundaries– Overrides MST ports at boundary– Triggers and Propagates Super Sync– May optionally carry some VLANs– Carries Region-Config and M-records
• MSTP is a much simplified 802.1w
