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 trac 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 congured 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 trac 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 trac 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 116. STP Loop Guard Prevents Forwarding Loops
836
Spanning Tree Protocol (STP)
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