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Multichassis Link Aggregation Group Overview


Multi-chassis link aggregation group (MC-LAG), is a type of LAG with constituent ports that terminate on separate chassis, thereby providing node-level redundancy. MC-LAG adds node-level redundancy to the normal link-level redundancy that a LAG provides. This allows two or more nodes to share a common LAG endpoint. The multiple nodes present a single logical LAG to the remote end.

The MC-LAG enables a client device to form a logical LAG interface between two MC-LAG peers. An MC-LAG provides redundancy and load balancing between the two MC-LAG peers, multi-homing support, and a loop-free Layer 2 network without running the Spanning Tree Protocol (STP). It is an HA solution involving multiple protocols: LACP, ICCP, VRRP, BFD, SNOOPING

On one end of an MC-LAG, there is an MC-LAG client device, such as a server, that has one or more physical links in a link aggregation group. This client device does not need to have an MC-LAG configured. On the other side of the MC-LAG, there are two MC-LAG peers. Each of the MC-LAG peers has one or more physical links connected to a single client device.

The MC-LAG peers use the Inter-chassis Control Protocol (ICCP) to exchange control information and coordinate with each other to ensure that data traffic is forwarded properly. ICCP runs over TCP and is monitored by Bidirectional Forwarding Detection (BFD). It requires an Inter-Chassis link (ICL) for L2 connectivity.

Following are the two types of MC-LAG:

  • Active-Active Mode—In active-active mode, all member links are active on the MC-LAG. In this mode, MAC addresses learned on one MC-LAG peer are propagated to the other MC-LAG peer. Active-active mode is the only mode supported at this time.

  • Active-Standby Mode—In active-standby mode, one node is active at any given time. In this mode, the MC-LAG peers act as virtual routers. The virtual routers share the virtual IP address that corresponds to the default route configured on the host or server connected to the MC-LAG.

You can configure MC-LAG in the following scenarios:

  • MC-LAG with multi-segmented point-to-point service in Active-Standby mode

  • MC-LAG with point-to-point service in Active-Standby mode

  • MC-LAG with multi-segmented Layer 3 VPN in Active-Standby mode