Reference Guide

Spanning Tree Protocol (STP) | 905
Configure LACP to be hitless using the command redundancy protocol lacp. Configure all Spanning Tree
types to be hitless using the command
redundancy protocol xstp from CONFIGURATION mode, as shown in
Figure 50-13.
Figure 50-13. Configuring all Spanning Tree Types to be Hitless
STP Loop Guard
STP Loop Guard is supported only on the S5000 switch.
Loop Guard Scenario
The STP Loop Guard feature provides protection against Layer 2 forwarding loops (STP loops) caused by
a hardware 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 be bidirectional) and an STP port
does not receive BPDUs. When an STP blocking port does not receive BPDUs, it transitions to a
forwarding state. This condition can create a loop in the network.
For example, in Figure 50-14 (STP topology 1 - upper left), Switch A is the root switch and Switch B
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 receive
BPDUs from Switch B. When the
max-age timer expires, the STP port on Switch C becomes unblocked
and transitions to forwarding state. A loop is created as both Switch A and Switch C transmit traffic to
Switch B.
Note that in Figure 50-14 (STP topology 2 - upper right), a loop can also be created if the forwarding 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 A and Switch C
transmit traffic to Switch B (STP topology 2 - lower right).
As shown in STP topology 3 (Figure 50-14 bottom middle), after you enable loop guard on an STP port or
port-channel on Switch C, if no BPDUs are received and the
max-age timer expires, the port transitions
from a blocked state to a loop-inconsistent state (instead of to a forwarding state). 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 loop guard on a port in a loop-inconsistent state, the port transitions to an STP
blocking state and restarts the
max-age timer.
Dell(conf)#redundancy protocol xstp
Dell#show running-config redundancy
!
redundancy protocol xstp
Dell#