MC-LAG

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MC-LAG, or Multi-Chassis Link Aggregation Group, is a type of LAG with constituent ports that terminate on separate chassis, thereby providing node-level redundancy. Unlike link aggregation in general, MC-LAG is not covered under IEEE 802.1AX-2008. Its implementation varies by vendor.

Link Aggregation Groups

A LAG (Link Aggregation Group) 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."[1] 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 defined within the 802.1AX-2008 standard.[2]

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 fail over 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.

MC-LAG

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 is vendor-specific; it is not covered by the IEEE 802.1AX-2008 standard. [3] Nodes in an MC-LAG cluster communicate to synchronize and negotiate automatic switchovers (failover). Some implementations may support administrator-initiated (manual) switchovers.

See also

  • LACP
  • Link aggregation
  • Virtual Link Trunking - a Dell proprietary implementation of MC-LAG
  • Virtual Port Channel - the Cisco (Nexus) and Dell Networking (DNOS6.x) proprietary implementations of MC-LAG

References

  1. ↑ IEEE. IEEE 802.1AX-2008. IEEE. 
  2. ↑ IEEE. IEEE 802.1AX-2008. IEEE. 
  3. ↑ Bhagat, Amit N. "Multichassis Link Aggregation Group". Google Knowledge Base. Retrieved 15 March 2012. 


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