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Multiple Spanning Trees

IEEE 802.1s. Multiple Spanning Trees. 05/21/2001 St. Louis Interim. Shyam Kaluve ( skaluve@cisco.com ). Common Spanning Tree (Now). Root. Common Spanning Tree (802.1s). Typical Deployment. A standalone 802.1s network. Old SST bridges in access layer. CST root in MST backbone.

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Multiple Spanning Trees

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  1. IEEE 802.1s Multiple Spanning Trees 05/21/2001 St. Louis Interim Shyam Kaluve (skaluve@cisco.com)

  2. Common Spanning Tree (Now) Root IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  3. Common Spanning Tree (802.1s) IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  4. 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 IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  5. MST Super Bridge IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  6. Switch fabric - MST instances Single Bridge vs Super Bridge • 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 IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  7. Single Bridge vs Super Bridge R Path Costs are not accurate IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  8. Z X Y Designated Bridge ID does not matter Single Bridge vs Super Bridge R A B C IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  9. 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 IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  10. Internal LAN LAN Lis internal to a bridge B if the designated bridge D for L also has the same Region Configuration as B Thanks to Mick IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  11. R MST Region IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  12. ISTP Holds Super Bridge together • Distributes CST info • Conveys MST Region Config • Identifies boundaries • Conveys Sync/ReRoot of Super Bridge IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  13. Super Sync IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  14. All MST ports are forcefully blocked Super Sync • 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 IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  15. Partial Super Sync IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  16. Current 7 component priority vector MSTP • 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 IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  17. Regular 4 component priority vector is sufficient MSTP • Operates the Super Bridge’s internal switch fabric • Totally unaffected by CST changes in a single 802.1s cloud IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  18. IST port state overrides M-ports Boundary Ports • All M-ports are edge ports • IST Root/Designated  MST Designated • IST Alternate  MST Overridden • IST Backup  MST Designated/Alternate • IST+MST  CST IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  19. If IST port is root port and becomes forwarding Rapid Transition At the boundary • Or if IST port is Designated and gets an agreement - then set agreement for all M-ports IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  20. {C-Root, C-Cost, C-Bridge, C-Port, I-Master, I-Cost, I-Bridge, I-Port} ISTP • Include CST Designated Bridge and Port ID to make an 8 component vector • Introduce IST Flags to indicate SuperSyncRequest IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  21. MST instances can use totally different algorithm Independence From CST • Change in Super Bridge’s root port does not necessarily cause a Super Sync • Separate hello-timer, forward-delay, max-age within the region (?) IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  22. 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 Conclusion • MSTP is a much simplified 802.1w IEEE 802.1s - St. Louis Interim - Shyam Kaluve

  23. IEEE 802.1s Multiple Spanning Trees 05/21/2001 St. Louis Interim Shyam Kaluve (skaluve@cisco.com)

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