Reference Guide

To disable STP root guard on a port or port-channel interface, use the no spanning-tree 0 rootguard command in an
interface configuration mode.
To verify the STP root guard configuration on a port or port-channel interface, use the show spanning-tree 0 guard
[interface interface] command in a global configuration mode.
Enabling SNMP Traps for Root Elections and
Topology Changes
To enable SNMP traps individually or collectively, use the following commands.
● Enable SNMP traps for spanning tree state changes.
snmp-server enable traps stp
● Enable SNMP traps for RSTP, MSTP, and PVST+ collectively.
snmp-server enable traps xstp
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 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 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 (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.
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Spanning Tree Protocol (STP)