Multi-Chassis LAG (MLAG)

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Multi-Chassis LAG (MLAG)

LAG a.k.a Link aggregation is a technology where multiple links from a switch chassis were bundled together and connected to multiple links on the neighbor switch chassis to provide increased bandwidth capacity. Multiple links combined act as one logical link and in case any physical link fails the traffic is automatically shifted onto another link in bundle hence providing some redundancy. However in LAG in case a switch chassis goes faulty then there is no redundancy.

MLAG is an enhancement of LAG where the ports from different switch chassis are bundled together to act as a single logical link. This also provides active/active uplinks with no layer 2 loops. MLAG switch is much similar to VPC feature in Cisco Nexus but MLAG being non-proprietary.


MLAG terminologies:

  • MLAG Domain – A pair of physical switches
  • MLAG peer – A single switch within an MLAG domain
  • MLAG peer-link – Ethernet physical link between MLAG peers required to synchronize state between MLAG peers and carry user data
  • MLAG member port – One or more user-facing physical ports that forms an MLAG
  • MLAG port-channel – This is a combined user-facing port-channel between MLAG peers consisting of MLAG member ports on both MLAG peers
  • MLAG VLAN – A VLAN used exclusively on the MLAG peer-link, in order to aid control-plane communication between MLAG peers.


Multi-Chassis LAG (MLAG) configuration steps are as below:

Step 1: Configure the peer link.

There should be minimum of two ports for redundancy and under normal circumstances peer link shouldn’t be used to carry any data-traffic.

Switch1(config)#interface eth 1-2

Switch1(config-if-Et1-2)#channel-group 10 mode active

Switch1(config-if-Et1-2)#interface port-channel 10

Switch1(config-if-Po10)#switchport mode trunk

Switch2(config)#interface eth 1-2

Switch2(config-if-Et1-2)#channel-group 10 mode active

Switch2(config-if-Et1-2)#interface port-channel 10

Switch2(config-if-Po10)#switchport mode trunk

Step 2: Create the Peer link VLAN

It is the VLAN used for MLAG TCP/UDP session. SVI should be configured on both peers and Spanning-tree needs to be disabled.

Switch1(config)#vlan 4094

Switch1(config-vlan-4094)#trunk group mlag

Switch1(config)#int vlan 4094

Switch1(config-if-Vl4094)#ip address

Switch1(config-if-Vl4094)#no autostate  #Required to bring the VLAN 4094 SVI up as it isnt a member of any physical port.

Switch1(config)#int port-channel 10

Switch1(config-if-Po10)#switchport trunk group mlag

Switch1(config)#no spanning-tree vlan 4094

Switch2(config)#vlan 4094

Switch2(config-vlan-4094)#trunk group mlag

Switch2(config)#int vlan 4094

Switch2(config-if-Vl4094)#ip address

Switch2(config-if-Vl4094)#no autostate    #Required to bring the VLAN 4094 SVI up as it isnt a member of any physical port.

Switch2(config)#int port-channel 10

Switch2(config-if-Po10)#switchport trunk group mlag

Switch2(config)#no spanning-tree vlan 4094

Step 3: Configure the MLAG domain


Switch1(config-mlag)#domain-id Admin

Switch1(config-mlag)#peer-link port-Channel 10

Switch1(config-mlag)#local-interface vlan 4094



Switch2(config-mlag)#domain-id Admin

Switch2(config-mlag)#peer-link port-Channel 10

Switch2(config-mlag)#local-interface vlan 4094


Step4: Create the MLAG port members

Switch1(config)#interface eth 3

Switch1(config-if-Et3)#channel-group 1 mode active

Switch1(config-if-Et3)#int port-Channel 1

Switch1(config-if-Po1)#switchport mode trunk

Switch1(config-if-Po1)#mlag 1


Switch1(config-if-Po1)#show port-channel 1

Port Channel Port-Channel1:

Active Ports: Ethernet3 PeerEthernet3

Switch2(config)#interface eth 3

Switch2(config-if-Et3)#channel-group 1 mode active

Switch2(config-if-Et3)#int port-Channel 1

Switch2(config-if-Po1)#switchport mode trunk

Switch2(config-if-Po1)#mlag 1


Switch2(config-if-Po1)#show port-channel 1

Port Channel Port-Channel1:

Active Ports: Ethernet3 PeerEthernet3

To verify the MLAG peer you can run the command show mlag detail

Related- Multi Chassis Trunking (MCT) Terminologies


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