The Arista MLS switch must have STP Loop Guard enabled on all nondesignated STP switch ports.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-255972	ARST-L2-000070	SV-255972r882258_rule		Medium

Description
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.

Description

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.

STIG	Date
Arista MLS EOS 4.2x L2S Security Technical Implementation Guide	2023-01-11

Details

Check Text ( C-59648r882256_chk )
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.

Fix Text (F-59591r882257_fix)
Configure the Arista MLS switch for STP Loop Guard globally with the following command: switch(config)#spanning-tree guard loop default switch(config)# Alternatively, configure Loop Guard on each interface: switch(config-if-Eth6)# spanning-tree guard loop