Allied Telesis AT-FS970M Series - Manual (Page 769)

AT-FS970M Switch Command Line User’s Guide
769
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 151. 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.