Allied Telesis AT-S63 - Manual (Page 283)

AT-S63 Management Software Features Guide
Section V: Spanning Tree Protocols 283
RSTP Loop Guard
Although RSTP is intended to detect and prevent the formation of loops in
a network topology, it is possible that the protocol might inadvertently
create a loop. This can happen in the unlikely situation where a link
between two RSTP devices remains active when there is an cessation of
BPDUs because of a hardware or software problem. This feature is
designed to prevent the formation of a loop in this situation.
Network devices running RSTP regularly transmit BPDUs to discover the
topology of a network and to the search for loops. These packets are used
by the devices to identify redundant physical paths to the root bridge and,
where loops exist, to determine the ports to be blocked.
The proper operation of RSTP relies on the flow of these packets. If there
is a hardware or software failure that interrupts their transmission or
reception, it is possible the protocol might mistakenly unblock one or more
ports in the spanning tree domain, causing a network loop.
The RSTP loop guard feature protects against this type of failure by
monitoring the ports on the switch for BPDUs from the other RSTP
devices. If a port stops receiving BPDUs without a change to its link state
(that is the link on a port stays up), the switch assumes that there is a
problem with RSTP on the other device and takes action depending on a
port’s role in the spanning tree domain. If the event happens on an
alternate port in the blocking state, the port is kept in that state. If this
occurs on a root or designated port in the forwarding state, the port’s state
is changed to the blocking state.
The switch activates loop guard only when there is a cessation in the flow
of BPDUs on a port whose link state has not changed. A port that never
receives BPDUs will not be affected by this feature.
A port that loop guard has placed in the blocking state remains in that state
until it begins to receive BPDUs again or you reset the switch.
Disconnecting the port, disabling or enabling a port with the management
software, or even disabling loop guard does not change a port’s blocking
state.
If a loop guard event occurs during a local or remote management
session, you will see this message displayed on the screen:
Loop Guard is triggered
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.
This event does not generate an entry in the switch’s log.