A multi-chassis link aggregation group (MLAG or MC-LAG) is a type of link aggregation group (LAG) with constituent ports that terminate on separate chassis, primarily for the purpose of providing redundancy in the event one of the chassis fails., After the FortiSwitch units are configured as MCLAG peer switches, any port that supports advanced features on the FortiSwitch can become a LAG port. When mclag is enabled and the LAG port names match, an MCLAG peer set is automatically formed., After the FortiSwitch units are configured as MCLAG peer switches, any port that supports advanced features on the FortiSwitch can become a LAG port. When mclag is enabled and the LAG port names match, an MCLAG peer set is automatically formed., 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., A multichassis LAG (MCLAG) provides node-level redundancy by grouping two FortiSwitch models together so that they appear as a single switch on the network. If either switch fails, the MCLAG continues to function without any interruption, increasing network resiliency and eliminating the delays associated with the Spanning Tree Protocol (STP)., Configure the FortiLink interface by adding the FortiGate port connected to FortiLink (for enabling FortiLink on any aggregate interface, it can only be done on FortiGate CLI, with ' set enable fortilink ' under system interface)..