Allied Telesis AT-8100 Series - Manual (Page 790)

Chapter 53: STP, RSTP and MSTP Protocols
790
If you configured the SNMP community strings on the switch, an SNMP
trap is sent to your management workstations to notify you of the event.
However, this event does not generate an entry in the switch’s log.
This feature is supported on the base ports of the switch as well as on any
fiber optic transceivers installed in the unit.
The following figures illustrate this feature. The first figure shows spanning
tree under normal operations in a network of three switches that have
been connected to form a loop. To block the loop, switch 3 designates port
14 as an alternate port and places it in the blocking or discarding state.
Figure 153. Loop Guard Example 1
If port 17 on switch 2 stops transmitting BPDUs, port 14 on switch 3
transitions from the blocking state to the forwarding state because the
switch assumes that the device connected to the port is no longer an
RSTP device. The result is a network loop, as illustrated in Figure 154 on
page 791.