Spanning Tree Protocol (STP) | 1017Configuring Spanning Trees as HitlessConfiguring Spanning Trees as Hitless is supported only on platforms: c eYou can configure Spanning Tree (STP), Rapid Spanning Tree (RSTP), Multiple Spanning Tree (MSTP),and Per-Vlan Spanning Tree (PVST+) to be hitless (all or none must be configured as hitless). Whenconfigured as hitless, critical protocol state information is synchronized between RPMs so that RPMfailover is seamless and no topology change is triggered.Configure LACP to be hitless using the command redundancy protocol lacp. Configure all Spanning Treetypes to be hitless using the command redundancy protocol xstp from CONFIGURATION mode, as shown inFigure 49-7.Figure 49-7. Configuring all Spanning Tree Types to be HitlessSTP Loop GuardSTP Loop Guard is supported only on platforms: c e sLoop Guard ScenarioThe STP Loop Guard feature provides protection against Layer 2 forwarding loops (STP loops) caused bya hardware 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 Figure 49-8 (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 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.Note that in Figure 49-8 (STP topology 2 - upper right), a loop can also be created if the forwarding port onSwitch B becomes busy and does not forward BPDUs within the configured forward-delay time. As aresult, the blocking port on Switch C transitions to a forwarding state, and both Switch A and Switch Ctransmit traffic to Switch B (STP topology 2 - lower right).FTOS(conf)#redundancy protocol xstpFTOS#show running-config redundancy!redundancy protocol xstpFTOS#
