V-255972

Discussion

The Spanning Tree Protocol (STP) loop guard feature provides additional protection against STP loops. An STP loop is created when an STP blocking port in a redundant topology erroneously transitions to the forwarding state. In its operation, STP relies on continuous reception and transmission of BPDUs based on the port role. The designated port transmits BPDUs, and the nondesignated port receives BPDUs. When one of the ports in a physically redundant topology no longer receives BPDUs, the STP conceives that the topology is loop free. Eventually, the blocking port from the alternate or backup port becomes a designated port and moves to a forwarding state. This situation creates a loop. The loop guard feature makes additional checks. If BPDUs are not received on a nondesignated port and loop guard is enabled, that port is moved into the STP loop-inconsistent blocking state.

Check Content

Review the Arista MLS switch configuration to verify that STP Loop Guard is enabled. It can be enabled globally or applied to an interface.

switch# sh run | sec spanning-tree
spanning-tree guard loop default

Or,

interface Ethernet6
    spanning-tree guard loop

If STP Loop Guard is not configured globally or on nondesignated STP ports, this is a finding.