AristaConfig534 downloads

Arista EOS MLAG Configuration

Multi-chassis Link Aggregation Group (MLAG) for Arista EOS switches: peer-link, keepalive (management), MLAG domain, and MLAG port-channels.

↓ example.txt

What is it?

Arista MLAG (Multi-chassis Link Aggregation) allows two Arista switches to act as a single logical switch for downstream devices. A server or switch can connect to both MLAG peers simultaneously using LACP — getting both redundancy and bandwidth. MLAG uses a peer-link (a high-bandwidth trunk between the two switches) to carry MLAG control traffic and forwarded frames. A keepalive link (usually the management interface) monitors peer health independently. Unlike Cisco vPC, MLAG supports active-active L3 forwarding — both peers can route simultaneously with the same SVI IP.

When to use

·Dual-homing servers or ToR switches to two distribution/aggregation switches for redundancy and bandwidth
·Replacing Cisco vPC in existing designs — MLAG is Arista's equivalent
·Data center top-of-rack (ToR) layer where every server needs dual uplinks
·Any scenario requiring active-active link redundancy without STP blocking ports

When NOT to use

·Three or more switches — MLAG only works between exactly two peers. Use VXLAN EVPN with ESI for multi-chassis redundancy across more than two switches.
·Pure L3 leaf-spine fabric — if routing to every host, MLAG is not needed. Use ECMP routing instead.

How it compares

vs Cisco vPC (NX-OS)Arista MLAG and Cisco vPC are functionally equivalent. Key differences: MLAG uses a single VLAN SVI for the keepalive (NX-OS uses a dedicated keepalive interface). MLAG supports active-active L3 routing more natively than vPC.

vs Juniper MC-LAGJuniper MC-LAG uses an 'inter-chassis link' (ICL) for peer-link. Same concept as Arista MLAG. Both provide active-active dual-homing for downstream devices.

Prerequisites

→A dedicated high-bandwidth peer-link (at least 10G, ideally 2x10G or 1x100G) between the two MLAG peers
→A dedicated VLAN for the MLAG peer-link SVI (MLAG keepalive)
→Both peers must run the same EOS version for feature compatibility
→MLAG domain ID must match on both peers; local and peer IP must be in the same /30 subnet

Config Generator

Hostname *

MLAG Domain ID *

MLAG Local IP (peer-link SVI) *

MLAG Peer IP *

MLAG Peer-link Netmask

MLAG Peer-link VLAN ID

Keepalive Source IP *

Keepalive Peer IP *

Keepalive VRF

Peer-Link Member Interfaces

MLAG Port-Channels

Reload-delay (seconds)

Verification commands

show mlag

Show MLAG domain status, peer health, and port-channel states

ExpectedState: 'active'. Peer-link: 'up'. Peer connectivity: 'up'. Config warnings: 'none'.

show mlag interfaces

Show all MLAG port-channels and their status on both peers

ExpectedEach MLAG ID shows 'active-full' state. 'Inactive' means the port-channel is down on one peer.

show port-channel summary

Verify LACP port-channel state and active member interfaces

ExpectedPort-channels show 'U' (in use) and 'P' (bundled) members. 'I' (individual) means LACP not negotiating.

Debug commands

show mlag detail

Show detailed MLAG state including config consistency check results

Common mistakes & fixes

SymptomMLAG shows 'inactive' — peers not forming

CausePeer-link port-channel not up, or MLAG domain ID / keepalive IP mismatch

FixVerify peer-link Port-Channel1000 is up: 'show interfaces Port-Channel1000'. Check domain-id, peer-address, and local-interface match on both peers.

SymptomMLAG port-channel shows 'partial' — only one side active

CauseThe port-channel is up on one peer but down on the other

FixCheck 'show mlag interfaces' for the specific MLAG ID. The 'partial' state means downstream device sees one link as individual (not bundled). Verify LACP is active on the downstream device.

SymptomTraffic not load-balancing across MLAG peers

CauseDownstream device only connected to one peer, or LACP hash too narrow

FixVerify downstream LACP port-channel has active members on both MLAG peers. Adjust LACP hash: 'port-channel load-balance trident fields ip dst-ip src-ip'

Further learning

Recommended books to go deeper on this topic.

BGP Design and Implementation

Randy Zhang & Micah Bartell · Cisco Press · BGP foundation for MLAG/EVPN environments on Arista EOS

Kindle ↗Hardcover ↗Paperback ↗

NX-OS and Cisco Nexus Switching: Next-Generation Data Center Architectures

Ron Fuller, David Jansen, Matthew McPherson · Cisco Press · MLAG/vPC concepts translate directly to Arista EOS — same design principles

Hardcover ↗