Administrator Guide

Configuring Root Guard
Enable STP root guard on a per-port or per-port-channel basis.
Dell Networking OS Behavior: The following conditions apply to a port enabled with STP root guard:
• Root guard is supported on any STP-enabled port or port-channel interface.
• Root guard is supported on a port in any Spanning Tree mode:
• Spanning Tree Protocol (STP)
• Rapid Spanning Tree Protocol (RSTP)
• Multiple Spanning Tree Protocol (MSTP)
• Per-VLAN Spanning Tree Plus (PVST+)
• When enabled on a port, root guard applies to all VLANs configured on the port.
• You cannot enable root guard and loop guard at the same time on an STP port. For example, if you configure root guard on a port on
which loop guard is already configured, the following error message displays: • % Error: LoopGuard is configured.
Cannot configure RootGuard.
• When used in an MSTP network, if root guard blocks a boundary port in the CIST, the port is also blocked in all other MST instances.
To enable the root guard on an STP-enabled port or port-channel interface in instance 0, use the following command.
• Enable root guard on a port or port-channel interface.
INTERFACE mode or INTERFACE PORT-CHANNEL mode
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
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).
Spanning Tree Protocol (STP)
907