Multi-chassis link aggregation group
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. The IEEE 802.1AX-2008 industry standard for link aggregation does not mention MC-LAG, but does not preclude it. Its implementation varies by vendor; notably, the protocol existing between the chassis is proprietary. Background. A LAG is a method of inverse multiplexing over multiple Ethernet links, thereby increasing bandwidth and providing redundancy. It is defined  by the IEEE 802.1AX-2008 standard, which states, "Link Aggregation allows one or more links to be aggregated together to form a Link Aggregation Group, such that a MAC client can treat the Link Aggregation Group as if it were a single link." This layer 2 transparency is achieved by the LAG using a single MAC address for all the device’s ports in the LAG group. LAG can be configured as either static or dynamic. Dynamic LAG uses a peer-to-peer protocol for control, called Link Aggregation Control Protocol (LACP). This LACP protocol is also defined within the 802.1AX-2008  standard. LAG can be implemented in two ways. LAG N and LAG N+N. LAG N is the load sharing mode of LAG and LAG N+N provides the worker standby flavour. The LAG N protocol automatically distributes and load balances the traffic across the working links within a LAG, thus maximising the use of the group if Ethernet links go down or come back up, providing improved resilience and throughput. For a different style of resilience between 2 nodes, a complete implementation of the LACP protocol supports separate worker/standby LAG subgroups. For LAG N+N, the worker links as a group  will failover to the standby links if any one or more or all of the links in the worker group fail. Note, LACP marks links as in standby mode using an "out of sync" flag. Multi-chassis. 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. Note that MC-LAG implementations are vendor-specific, but cooperating chassis remain externally compliant to the IEEE 802.1AX-2008 standard. Nodes in an MC -LAG cluster communicate to synchronize and negotiate automatic switchovers (failover). Some implementations may support administrator-initiated (manual) switchovers. The diagram here shows four configurations: The HA configuration is superior to spanning tree. Load can be shared across all links during normal operation, whereas spanning tree must disable some links to prevent loops. Implementations. The following table lists known vendor implementations of MC-LAG, all of which are proprietary. Alternatives. IEEE 802.1aq (Shortest Path Bridging) is an alternative to MC-LAG that can be used for complex networks. TRILL (TRansparent Interconnection of Lots of Links) facilitates an Ethernet to have an  arbitrary topology, and enables per flow pair-wise load splitting by way of Dijkstra's algorithm, without configuration and user intervention.