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Chapter 40: STP, RSTP and MSTP Protocols
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STP, RSTP, MSTP Loop Guard
Although spanning tree is designed to detect and prevent the formation of
loops in a network topology, it is possible in certain circumstances for the
protocol to inadvertently create loops. This can happen in the unlikely
situation where a link between two spanning tree devices remains active
when there is an cessation of BPDUs because of a hardware or software
problem. The loop guard feature is designed to prevent the formation of
loops in this situation.
Note
The Loop Guard feature is supported in STP, RSTP, and MSTP.
Network devices running spanning tree 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 spanning tree 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 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