Скачать или смотреть Loop Guard

Cisco has added two features to detect or prevent the unexpected loss of BPDU'
UDLD and Loop Guard

Suppose a switch port which is neither designated nor the root port which means its a
non-designated or a blocking port

Assume if that port is not receiving the BPDU for some unknown reason ( it can be a cabling issues)

In that switch port will wait for the Max-Age timer to expire , it will flush the BPDU from the memory and moves the current port (blocking port ) into forwarding Port which will lead to a switching loop

To prevent this situation we are enabling loop guard which main function is to track the sudden loss of BPDU on a non-designated port