Multiple Spanning Trees

1
Multiple Spanning Trees
IEEE 802.1s
05/21/2001 St. Louis Interim

• Shyam Kaluve (skaluve_at_cisco.com)

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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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Single Bridge vs Super Bridge
R
Path Costs are not accurate
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Single Bridge vs Super Bridge
R
A
B
C
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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
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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

• Operates the Super Bridges internal switch fabric

• Regular 4 component priority vector is sufficient

• Totally unaffected by CST changes in a single
  802.1s cloud

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Boundary Ports

• All M-ports are edge ports

• IST port state overrides M-ports

• IST Root/Designated ? MST Designated

• IST Alternate ? MST Overridden

• IST Backup ? MST Designated/Alternate

• ISTMST ? CST

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Rapid Transition
At the boundary

• If IST port is root port and becomes forwarding

• Or if IST port is Designated and gets an
  agreement

- then set agreement for all M-ports
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ISTP

• Include CST Designated Bridge and Port ID to make
  an 8 component vector

• C-Root, C-Cost, C-Bridge, C-Port, I-Master,
  I-Cost, I-Bridge, I-Port

• Introduce IST Flags to indicate SuperSyncRequest

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Independence From CST

• MST instances can use totally different algorithm

• Change in Super Bridges root port does not
  necessarily cause a Super Sync

• Separate hello-timer, forward-delay, max-age
  within the region (?)

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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

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Multiple Spanning Trees
IEEE 802.1s
05/21/2001 St. Louis Interim

• Shyam Kaluve (skaluve_at_cisco.com)