Arista EOS MLAG: Multi-Chassis Link Aggregation Explained

In a traditional dual-homed data center design, Spanning Tree Protocol blocks half of the uplinks to prevent loops — which means you pay for bandwidth you cannot use. Arista EOS MLAG (Multi-Chassis Link Aggregation) solves this by making two physical switches behave as one logical switch, so every uplink forwards traffic simultaneously. This article explains the MLAG building blocks — peer link, peer address, domain ID and the MLAG System ID — and shows the core configuration and verification commands from the official EOS user manual.

How MLAG Works

Two switches in an MLAG domain connect through a peer link (a dedicated port-channel carrying control traffic and, when needed, data from single-attached devices). Each peer uses the peer address to form and maintain that link. The MLAG domain ID is a text string both switches must share, and the MLAG System ID (MSI) is the domain's MAC address, automatically derived when MLAG forms — it appears in STP and LACP PDUs so downstream devices see one switch, not two.

Configuration Essentials

VLAN parameters must be identical on both peers for the LAGs that make up the peer link and MLAGs: access VLAN, switchport mode, trunk-allowed VLANs, native VLAN and trunk groups. Configuration drift between peers is the number one cause of traffic loss in failure scenarios.

LACP should be used on all MLAG interfaces, including the peer link — LACP control packets reference the MLAG system ID, and LACP-based LAGs automatically deactivate miscabled ports instead of bridging traffic erroneously.

switch1(config)# interface ethernet 3-4
switch1(config-if-Et3-4)# channel-group 20 mode active
switch1(config)# interface port-channel 20
switch1(config-if-Po20)# mlag 20

Verification Commands

Confirm MLAG health with the show mlag family of commands:

switch# show mlag
MLAG State:           active
Peer Link Status:     up
Peer Keepalive:       ok
MLAG Ports:           6 active, 0 inactive

Other useful variants include show mlag interfaces (per-port-channel MLAG status), show mlag peer-link (peer-link health), and show mlag issu warnings (upgrade compatibility before an ISSU).

Why It Matters

MLAG provides Layer 2 multipathing with sub-second failover: when one peer or one link fails, the remaining paths keep forwarding and the peer keeps MAC/ARP state synchronized. It is the foundation of most Arista leaf-spine designs. See 叶脊数据中心网络架构设计 and Arista EOS CLI 模式指南 for more on Arista architectures and CLI foundations.

原文链接：https://www.arista.com/en/um-eos/eos-multi-chassis-link-aggregation