Loop Guard

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/configuration 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 a 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 inconsistent blocking state, rather than
transitioning to a 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:
Switch(config)#spanning-tree loopguard default
To enable Loop Guard on a specific interface, use:
Switch(config-if)#spanning-tree guard loop
Loop Guard automatically re-enables the port if it starts receiving BPDUs
once again.