STP Loop GuardSTP loop guard is supported only on the platform.The STP loop guard feature provides protection against Layer 2 forwarding loops (STP loops) caused by ahardware failure, such as a cable failure or an interface fault. When a cable or interface fails, aparticipating STP link may become unidirectional (STP requires links to be bidirectional) and an STP portdoes not receive BPDUs. When an STP blocking port does not receive BPDUs, it transitions to aForwarding state. This condition can create a loop in the network.For example, in the following example (STP topology 1, upper left), Switch A is the root switch and SwitchB normally transmits BPDUs to Switch C. The link between Switch C and Switch B is in a Blocking state.However, if there is a unidirectional link failure (STP topology 1, lower left), Switch C does not receiveBPDUs from Switch B. When the max-age timer expires, the STP port on Switch C becomes unblockedand transitions to Forwarding state. A loop is created as both Switch A and Switch C transmit traffic toSwitch B.As shown in the following illustration (STP topology 2, upper right), a loop can also be created if theforwarding port on Switch B becomes busy and does not forward BPDUs within the configuredforward-delay time. As a result, the blocking port on Switch C transitions to a forwarding state, andboth 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-channelon Switch C, if no BPDUs are received and the max-age timer expires, the port transitions from a blockedstate to a Loop-Inconsistent state (instead of to a Forwarding state). Loop guard blocks the STP port sothat 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 blockingstate. If you disable STP loop guard on a port in a Loop-Inconsistent state, the port transitions to an STPblocking state and restarts the max-age timer.792 Spanning Tree Protocol (STP)
