lower left), Switch C does not receive BPDUs from Switch B. When the max-age timer expires, the STP port on Switch C becomesunblocked 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 becomesbusy and does not forward BPDUs within the configured forward-delay time. As a result, the blocking port on Switch C transitions to aforwarding 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 arereceived and the max-age timer expires, the port transitions from a blocked state to a Loop-Inconsistent state (instead of to a Forwardingstate). 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 loopguard 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 Loops934 Spanning Tree Protocol (STP)
