Arista EOS MLAG Configuration: Complete Guide

2026年8月25日 LOG admin

Multi-Chassis Link Aggregation (MLAG) is the technology that lets two Arista switches act as a single logical switch for dual-homed servers and switches, eliminating the 50% bandwidth penalty that Spanning Tree imposes on redundant topologies. This guide, drawn from the official Arista EOS user manual, explains how MLAG operates, the peer-link and MLAG system ID concepts, the configuration considerations that keep VLANs, LACP and STP consistent across peers, and the exact show commands used to verify an active MLAG domain. It is an essential reference for any data center engineer running Arista EOS.

What Is MLAG and Why Use It?

Arista switches support MLAG to logically aggregate ports across two switches. Two 10-gigabit ports, one from each MLAG-configured switch, connect to a host and appear as a single 20-gigabit link. MLAG provides Layer 2 multipathing, higher bandwidth utilization, and active-active redundancy compared with traditional Spanning Tree-governed designs. It also interoperates with static LAG or LACP on the attached devices without proprietary protocols.

MLAG Operation and Components

The two cooperating switches are called MLAG peer switches and communicate through an interface called the peer link. The peer link carries MLAG control information and any traffic from devices attached to only one peer. Each peer uses the peer address to form and maintain the link. The MLAG domain ID is a text string configured on each peer; the MLAG System ID (MSI) is the domain MAC address, derived automatically when MLAG forms, and is used in STP and LACP PDUs.

Configuration Considerations

VLANs and LACP Consistency

VLAN parameters (access VLAN, switchport mode, trunk-allowed VLANs, native VLAN, trunk groups) must be configured identically on both peers for the peer-link and MLAG LAGs. LACP should be used on all MLAG interfaces including the peer link, since LACP control packets reference the MLAG system ID. Configuration discrepancies cause traffic loss in certain failure scenarios.

Spanning Tree

STP must be configured globally and on port-channels with an MLAG ID. Port-specific STP settings (PortFast, BPDU Guard, BPDU filter) come from the switch where the port physically resides.

Control-Plane ACL Requirements

Any custom control-plane ACL applied to an MLAG port must include these rules, or MLAG will fail to establish:

permit tcp any any eq mlag ttl eq 255
permit udp any any eq mlag ttl eq 255
permit ip any any tracked

Do not remove permit ip any any tracked - its absence prevents MLAG from establishing.

Verifying MLAG with Show Commands

switch1# show mlag
switch1# show mlag interfaces
switch1# show mlag interfaces detail
switch1# show spanning-tree vlan-id 3903
switch1# show spanning-tree blocked
switch1# show port-channel

Healthy peers show state: Active with peer-link status Up and MLAG ports in Active-full state. In STP output, MLAG interfaces appear as a single entry; remote interfaces are prefixed with P (Peer). A blocked-port count of zero confirms the MLAG domain created no topology loops.

MLAG Maintenance and Split-Brain Protection

When a peer reboots, non-peer-link ports stay in errdisabled for the reload-delay period (300s on fixed switches, up to 1800s on modular platforms). Severing the peer-link cable can cause a split-brain state where each peer independently runs STP to prevent loops. MLAG ISSU lets you upgrade EOS on one peer with minimal disruption when versions are compatible.