Administrator Guide

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.
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 128. STP Loop Guard Prevents Forwarding Loops
Spanning Tree Protocol (STP)
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