Allied Telesis AT-8100 Series - Manual (Page 792)

Chapter 53: STP, RSTP and MSTP Protocols
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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
is not using loop guard, it continues to forward traffic on port 4. But since
no BPDUs are received on the port, it assumes that the device connected
to the port is not an RSTP device. Since switch 2 becomes the new root
bridge, port 14 on switch 3 transitions to the forwarding state from the
blocking state to become the new root port for the switch. The result is a
network loop.
Figure 156. Loop Guard Example 4
But if loop guard is active on port 4 on switch 3, the port is placed in the
blocking state since the reception of BPDUs is interrupted. This blocks the
loop. The port remains in the blocking state until it again receives BPDUs
or the switch is reset.