Administrator Guide

spanning-tree {0 | mstp | rstp | pvst} rootguard
• 0: enables root guard on an STP-enabled port assigned to instance 0.
• mstp: enables root guard on an MSTP-enabled port.
• rstp: enables root guard on an RSTP-enabled port.
• pvst: enables root guard on a PVST-enabled port.
To disable STP root guard on a port or port-channel interface, use the no spanning-tree 0 rootguard command in an interface
conguration mode.
To verify the STP root guard conguration on a port or port-channel interface, use the show spanning-tree 0 guard
[interface interface] command in a global conguration 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 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)
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