Loop Guard

Loop Guard is one of the guarding protocols to prevent a network from getting switching loops.

Assume that port A from switch C receives BPDU's from port AA from switch B. Suddenly, a problem appears and makes switch B unable to send BPDU from switch C. This activates the MaxAge time counter on port A switch C and after the counter expires, port A on switch C will open its port from BLK to FWD mode and sends a BPDU to switch B. When loop guard is applied, this occurence will never happen because when the switch's port does not receive BPDU frames, it will put its port to a "loop inconsistent mode" (which is still in a BLK state). After port A switch C receives BPDU's again from switch B, the port returns back from loop inconsistent BLK to normal BLK mode.

The configuration command for loop guard is done in the switch's configuration mode which is "spanning-tree loopguard default".