Users Guide

• Congure all spanning tree types to be hitless.
CONFIGURATION mode
redundancy protocol xstp
Example of Conguring all Spanning Tree Types to be Hitless
Dell(conf)#redundancy protocol xstp
Dell#show running-config redundancy
!
redundancy protocol xstp
Dell#
STP Loop Guard
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 the following example (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 trac
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
becomes busy and does not forward BPDUs within the congured 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 trac 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
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 trac 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.
Spanning Tree Protocol (STP)
853