Example of Configuring all Spanning Tree Types to be HitlessDell(conf)#redundancy protocol xstpDell#show running-config redundancy!redundancy protocol xstpDell#STP Loop GuardThe STP loop guard feature provides protection against Layer 2 forwarding loops (STP loops) caused by a hardware failure, such as a cablefailure 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, it transitions to a Forwardingstate. 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 B normally transmits BPDUs toSwitch 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 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.876 Spanning Tree Protocol (STP)
