Allied Telesis AT-FS970M Series - Manual (Page 768)

Chapter 51: STP, RSTP and MSTP Protocols
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Figure 149. Loop Guard Example 2
But if loop guard is enabled on port 14 on switch 3, the port, instead of
changing to the forwarding state, stays in the blocking state, preventing
the formation of the loop.
Figure 150. Loop Guard Example 3
The previous example illustrates how loop guard works to maintain a loop-
free topology by keeping alternate ports in the blocking state when they
stop receiving BPDUs. Loop guard can also work on root and designated
ports that are in the forwarding state. This is illustrated in the next two
examples.
In the first example, the root bridge stops transmitting BPDUs. If switch 3