Saturday, August 1, 2009

Little more spanning tree...

So, just a bit more I promise.
Unidirectional link detection
This is mostly for fiber optics, but is concerned with transmissions ONLY being able to be sent one way over a link (i.e. switch 1 can send to switch 2 over Po1, but switch 2 can no longer send anything over Po1 to switch 1)
To configure: (use ios help)
(config)udld enable
(interface)udld port [aggressive]
Aggressive mode will shut the port down if it has not detected a 8 UDLD hellos.

This feature will turn an stp enabled port from blocking to loop-inconsistent state if packets (bpdu's) are not recieved. It stops switching loops because in this instance (such as with udld) the switch that is no longer recieving bpdu's would go from blocking to forwarding, causing it to send packets over a link that was not indeed dead (this usually occurs because bpdu's have stop being received on a port other than the blocking port; therefore causing the transition from blocking to fowarding).
To configure:
(config)spanning-tree loopguard default
(interface)spanning-tree guard loop

BPDU skew detection
This is a DETECTION mechanism for BPDU's that are not being relayed as quickly as they should be (it will log syslog messages). BPDU's should be recieve approx. every 2 seconds by every participating switch. If enabled, this feature will notify the proper personal of the issue; it will NOT take any action
(config)set spantree bpdu-skewing enable

Rapid Spanning Tree Protocol 802.1w (STP 802.1d)
Uses alternate and backup ports to speed convergence times
  1. discarding (instead of disabled, blocking, and listening)
  2. learning
  3. forwarding
In RSTP every switch sends out bpdu's every 2 seconds. After 3 missed bpdu's the switch will consider a link down. This drops convergence time from 50 to 6 seconds!

PVST is per-vlan spanning tree and is cisco proprietary. It takes a little extra cpu and memory to run, but is great in that it allows to to run a seperate instance of STP per vlan in you network!
(Also have RPVST; you guess the name!)

Honorable mentions (probably need to hit these again later)
CST (common spanning tree)
MST(multiple spanning tree)

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