Specifications

CHAPTER 3
Spanning Tree
To control UDLD on a specific fiber port, use the following command:
(config-if)# udld port {aggressive | disable}
To reenable all interfaces shut by UDLD, use the following:
# udld reset
To verify UDLD status, use the following:
# show udld interface
Loop Guard
Loop Guard prevents loops that might develop if a port that should be blocking inadvertently transitions to the forwarding
state. This can happen if the port stops receiving BPDUs (perhaps because of a unidirectional link or a software/configu-
ration problem in its neighbor switch). When one of the ports in a physically redundant topology stops receiving BPDUs,
the STP conceives the topology as loop-free. Eventually, the blocking port becomes designated and moves to forwarding
state, thus creating a loop. With Loop Guard enabled, an additional check is made.
If no BPDUs are received on a blocked port for a specific length of time, Loop Guard puts that port into “loop inconsis-
tent” blocking state, rather than transitioning to forwarding state. Loop Guard should be enabled on all switch ports that
have a chance of becoming root or designated ports. It is most effective when enabled in the entire switched network in
conjunction with UDLD.
To enable Loop Guard for all point-to-point links on the switch, use the following command:
(config)# spanning-tree loopguard default
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CCNP SWITCH 642-813 Quick Reference by Denise Donohue
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