What is Multi Chassis Link Aggregation (MC-LAG)?

What is Multi Chassis Link Aggregation (MC-LAG)?

The 802.3ad Link Aggregation Protocol standard provides a mechanism to aggregate multiple links to form one logical link. LACP is responsible for many functions; including link ordering, assigning active and standby links, detecting far-end link failures, etc. 802.3ad has gained momentum and is currently widely deployed and supported however it has one major shortfall, it does not have any provisions to allow the links in the Link Aggregation Group to span more than one end-device (switch/server).

To address this gap and provide an ultimate resiliency solution in terms of simplicity, scalability, convergence, Nortel developed an extension to the Link Aggregation standard. This extension is called Split-MLT (SMLT). Today most of the large Ethernet switch vendor supports multi-chassis link aggregation technology on their core switches to achieve five Nine of resiliency in the network core. Every vendor uses its own way of achieving it, few maintain completely separate control plane, however, few vendors maintain single control plane between the dual core switches running their own MC-LAG technology.

This implementation brings to end the use of STP protocol in the core of the network by providing active-active core switch deployment and faster convergence time.

Technology methodology

MC-LAG logically aggregates two Core Switches to form one logical entity where these two switches connected by some kind of trunk. This very trunk is used to exchange forwarding and routing databases between the two peer nodes and is completely transparent to access switches/devices. Traffic is automatically balanced across the links and nodes using a standard hashing algorithm eliminating any packet duplication. Deterministic hashing eliminates any packet duplication hence eliminating the need for spanning tree.

MC-LAG Benefits

  • STP-free Core Network resulting fast link or per failover (ms failover)
  • Technology transparent to Access devices, so access device can be from any vendor and any type. For example access device can be a firewall, switch, server etc
  • Link and Device-level network resiliency
  • No Bandwidth waste and single point of failure in the core of the network


Technology adoptions by various vendors

But this was not the end; Vendors have gone ahead and tried to solve few problems with MC-LAG technology. Few challenges see with the technology are

  • Proprietary to OEM/Vendor
  • LAG limited to 2/4 devices only
  • Lot of manual configuration for setup
  • Split brain situation (link between core switches fail), sometimes lead to network outage
  • Limited to single location and direct connectivity between device

Few vendors have identified these challenges and worked out with various solutions using Ethernet Multipath technologies. Kindly visit Ethernet Multipath Technology article to know more.

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