Administrator Guide

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.
As shown in the following illustration (STP topology 2, upper right), a loop can also be created if the forwarding port on Switch
B becomes busy 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 (STP topology 2, lower
right).
As shown in STP topology 3 (bottom middle), after you enable loop guard on an STP port or port-channel on Switch C, if no
BPDUs are 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 142. STP Loop Guard Prevents Forwarding Loops
Configuring Loop Guard
Enable STP loop guard on a per-port or per-port channel basis.
The following conditions apply to a port enabled with loop guard:
● Loop guard is supported on any STP-enabled port or port-channel interface.
● Loop guard is supported on a port or port-channel in any spanning tree mode:
Spanning Tree Protocol (STP)
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