snmp-server enable traps xstpSTP Loop GuardThe 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, a participating STP link may become unidirectional (STP requires links to bebidirectional) and an STP port does not receive BPDUs. When an STP blocking port does not receive BPDUs, ittransitions to a Forwarding 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 Switch Bnormally 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 receive BPDUsfrom Switch B. When the max-age timer expires, the STP port on Switch C becomes unblocked andtransitions 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 theforwarding 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 Aand 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 onSwitch C, if no BPDUs are received and the max-age timer expires, the port transitions from a blocked stateto a Loop-Inconsistent state (instead of to a Forwarding state). Loop guard blocks the STP port so that notraffic is transmitted and no loop is created.Spanning Tree Protocol (STP) 1088
