Loop Guard

As its name implies, Loop Guard is a method for ensuring that STP loops never occur in a particular topology. Even though STP guards against such loops as best it can, they could still occur because of things like unidirectional link failures or switch congestion issues. Loop Guard prevents loops conservatively by preventing alternate or root ports from becoming DPs in the topology. If BPDUs are not received on a non-DP, and Loop Guard is enabled, that port is moved into the STP loop-inconsistent Blocking state, instead of the Listening / Learning / Forwarding state. Loop Guard operates only on ports that are considered point-to-point by the spanning tree, and it cannot be run in conjunction with Root Guard on an interface.

To enable Loop Guard, you can use the following global configuration mode command:

spanning-tree loopguard default