Reference Guide

Spanning Tree Protocol | 995
STP Loop Guard
STP Loop Guard is supported only on platforms: c e
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Loop Guard Scenario
The STP Loop Guard feature provides protection against Layer 2 forwarding loops (STP loops) caused by
a hardware failure, such as a cable failure or an interface fault. When a cable or interface fails, a
participating STP link may become unidirectional (STP requires links to be bidirectional) and an STP port
does not receive BPDUs. When an STP blocking port does not receive BPDUs, it transitions to a
forwarding state. This condition can create a loop in the network.
For example, in Figure 55-5 (upper left) Switch A is the root switch and Switch B normally transmits
BPDUs to Switch C. However, if there is a unidirectional link failure (lower left), Switch C does not
receive BPDUs from Switch B. When the
max-age timer expires, the STP port on Switch C becomes
unblocked and transitions to forwarding state. A loop is created as both Switch A and Switch C transmit
traffic to Switch B (lower left).
A loop can also be created if the forwarding port on Switch B becomes busy (Figure 55-5 upper right) and
does not forward BPDUs within the configured
forward-delay time. As a result, the blocking port on
Switch C transitions to a forwarding state, and both Switch A and Switch C transmit traffic to Switch B
(lower right).
After you enable loop guard on an STP port or port-channel, if no BPDUs are not received and the
max-age timer expires, the port transitions from a blocked state to a loop-inconsistent state (instead of to a
forwarding state). Loop guard blocks the STP port so that no traffic is transmitted and no loop is created.
As soon as a BPDU is received on an STP port in a loop-inconsistent state, the port returns to a blocking
state. If you disable STP loop guard on a port in a loop-inconsistent state, the port transitions to an STP
blocking state and restarts the
max-age timer.
Figure 55-5. STP Loop Guard Example