How to Configure MC-LAG (Multi-Chassis Link Aggregation)
MC-LAG (Multi-Chassis Link Aggregation)
Objective
This document explains how to configure MC-LAG (Multi-Chassis Link Aggregation) between two switches and a host. MC-LAG provides link redundancy and load balancing by allowing two physical switches to appear as a single logical LAG (Link Aggregation Group) to the connected host, improving resiliency and eliminating single points of failure.
Prerequisites
Hardware:
Switches or routers that support VLANs
(e.g., edgecore, ufispace)
Software:
Network operating systems that support VLAN configuration
(e.g., SONiC-OS-4.4.0-Enterprise_Base, SONiC-OS-4.4.2-Enterprise_Base)
Access:
Administrative or privileged CLI access
Network:
Physical connections between Switch 1 and Switch 2 (peer-link or ICCP/ISL)
Physical LAG (bonded) links from Switches to the host
Topology Diagram
Example:
Two switches connected via peer link, both uplinked to a single host using port-channel bonds for redundancy and load balancing.
Basic Configuration Steps
1. Configure Peer Link Between Switches
Establish a dedicated link between Switch 1 and Switch 2 for control plane synchronization.
2. Enable MC-LAG on Both Switches
Configure the IP address on the dedicated management Ethernet port under the mgmt VRF.
3. Configure MC-LAG Interface
Define MC-LAG interface (logical LAG) and assign member ports.
4. Configure LAG on Host
Set up LACP bonding on the host to aggregate connections from both switches.
Example Configuration (DUT_1 and DUT_2)
DUT_1
configure terminal
interface PortChannel3
switchport trunk allowed Vlan 100,200
no shutdown
!
interface PortChannel1
switchport access Vlan 100
mclag 1
exit
interface Ethernet47
channel-group 3
!
interface Ethernet0
channel-group 1
!
interface Vlan200
description peer-link_vlan
ip address 192.168.1.1/24
!
interface Vlan100
ip anycast-address 10.10.10.1/24
!
mclag domain 1
source-ip 192.168.1.1
peer-ip 192.168.1.2
peer-link PortChannel3
mclag-system-mac 02:11:22:33:44:55
keepalive-interval 1
session-timeout 30
delay-restore 300
fast-convergence
DUT_1
configure terminal
interface PortChannel3
switchport trunk allowed Vlan 100,200
no shutdown
!
interface PortChannel1
switchport access Vlan 100
mclag 1
exit
interface Ethernet47
channel-group 3
!
interface Ethernet0
channel-group 1
!
interface Vlan200
description peer-link_vlan
ip address 192.168.1.2/24
!
interface Vlan100
ip anycast-address 10.10.10.1/24
!
mclag domain 1
source-ip 192.168.1.2
peer-ip 192.168.1.1
peer-link PortChannel3
mclag-system-mac 02:11:22:33:44:55
keepalive-interval 1
session-timeout 30
delay-restore 300
fast-convergence
Verification
show mclag brief #shows mclag status
show PortChannel summary #shows portchannel and lacp status
show vlan #Check vlan status
Show interface status #check all connected ports are up
Troubleshooting Tips
Ensure peer-link is up and stable.
Verify MC-LAG domain IDs match on both switches.
Check LACP configuration on both switches and host.
Confirm interface members are active in the port-channel.
Look for split-brain or orphan-port warnings if peer-link fails.
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