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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

IEEE 802.1s

Multiple Spanning Trees

05/21/2001 St. Louis Interim

Shyam Kaluve (skaluve@cisco.com)

common spanning tree now
Common Spanning Tree (Now)

Root

IEEE 802.1s - St. Louis Interim - Shyam Kaluve

common spanning tree 802 1s
Common Spanning Tree (802.1s)

IEEE 802.1s - St. Louis Interim - Shyam Kaluve

typical deployment
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

mst super bridge
MST Super Bridge

IEEE 802.1s - St. Louis Interim - Shyam Kaluve

single bridge vs super bridge
Switch fabric - MST instancesSingle 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

single bridge vs super bridge7
Single Bridge vs Super Bridge

R

Path Costs are not accurate

IEEE 802.1s - St. Louis Interim - Shyam Kaluve

single bridge vs super bridge8

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

mst region
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

internal lan
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

mst region11

R

MST Region

IEEE 802.1s - St. Louis Interim - Shyam Kaluve

slide12
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

super sync
Super Sync

IEEE 802.1s - St. Louis Interim - Shyam Kaluve

super sync14
All MST ports are forcefully blockedSuper 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

partial super sync
Partial Super Sync

IEEE 802.1s - St. Louis Interim - Shyam Kaluve

slide16
Current 7 component priority vectorMSTP
  • 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

slide17
Regular 4 component priority vector is sufficientMSTP
  • 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

boundary ports
IST port state overrides M-portsBoundary 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

rapid transition
If IST port is root port and becomes forwardingRapid 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

slide20
{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

independence from cst
MST instances can use totally different algorithmIndependence 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

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

Conclusion
  • MSTP is a much simplified 802.1w

IEEE 802.1s - St. Louis Interim - Shyam Kaluve

multiple spanning trees23

IEEE 802.1s

Multiple Spanning Trees

05/21/2001 St. Louis Interim

Shyam Kaluve (skaluve@cisco.com)

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