Multi-chassis link aggregation group

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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 for coordination between chassis is proprietary.

Contents

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." [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, called Link Aggregation Control Protocol (LACP), for control. This LACP protocol is also defined within the 802.1AX-2008 standard.

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. [2] 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:

Illustration comparing LAG to high-availability MLAG Using Multi-Chassis LAG (MC-LAG) for High Availability.png
Illustration comparing LAG to high-availability MLAG
  1. Switches A and B are each configured to group four discrete links (as indicated in green) into a single logical link with four times the bandwidth. Standard LACP protocol ensures that if any of the links go down, traffic will be distributed among the remaining three.
  2. Switch A is replaced by two chassis, switches A1 and A2. They communicate between themselves using a proprietary protocol and are thereby able to masquerade as a single "virtual" switch A running a shared instance of LACP. Switch B is not aware that it is connected to more than one chassis.
  3. Switch B is also replaced by two chassis B1 and B2. If these switches are from a different vendor, they may use a different proprietary protocol between themselves. But "virtual" switches A and B still communicate using LACP.
  4. Crossing two links to form an X makes no difference logically, any more than crossing links in a normal LAG would. However, physically it provides much improved fault tolerance (high availability). If any of the switches fail, LACP reconfigures paths in as little as a few seconds. Operation continues with paths existing between all sources and destinations, albeit with degraded bandwidth.

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.

VendorImplementation Name
ADVA Optical Networking MC-LAG
Arista Networks MLAG
Aruba Networks (formerly HP ProCurve)Distributed Trunking under Intelligent Resilient Framework switch clustering technology
Avaya Distributed Split Multi-Link Trunking
Ruckus Networks (formerly Brocade)Multi-Chassis Trunking
Ciena MC-LAG
Cisco Catalyst 6500Multichassis Etherchannel (MEC) - Virtual Switching System (VSS)
Cisco Catalyst 3750 (and similar)Cross-Stack EtherChannel
Cisco Catalyst 9000StackWise Virtual
Cisco Nexus Virtual PortChannel (vPC), where a PortChannel is a regular LAG
Cisco IOS XRmLACP (Multichassis Link Aggregation Control Protocol)
Cumulus Networks MLAG (formerly CLAG)
Dell Networking (formerly Force10 Networks, formerly nCore)DNOS6.x Virtual Port Channel (vPC) or Virtual Link Trunking
Edgecore Networks MLAG
Extreme Networks MLAG (Multi Switch Link Aggregation Group)
Ericsson MC-LAG (Multi Chassis Link Aggregation Group)
FS MLAG
Fortinet MC-LAG (Multi Chassis Link Aggregation Group)
H3C Distributed Resilient Network Interconnect
Huawei M-LAG
Juniper MC-LAG
Lenovo Networking (formerly IBM)vLAG
Mellanox Technologies MLAG
MikroTik MLAG [3]
NEC MC-LAG (Openflow to traditional network)
NocsysMLAG
Netgear MLAG
Nokia (Formerly Alcatel-Lucent)MC-LAG
Nortel Split multi-link trunking
Nuage Networks (from Nokia)MC-LAG ; including MCS (Multi-chassis Sync)
Plexxi (now Aruba Networks))vLAG
Pluribus Networks (now Arista Networks)vLAG
UniFi MC-LAG [4]
ZTE MC-LAG

Alternatives

IEEE 802.1aq (Shortest Path Bridging) is an alternative to MC-LAG that can be used for complex networks. [5]

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.

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References

  1. IEEE. IEEE 802.1AX-2008. IEEE.
  2. Bhagat, Amit N. "Multichassis Link Aggregation Group". Google Knowledge Base. Retrieved 15 March 2012.
  3. MikroTik: Multi-chassis Link Aggregation Group
  4. (Enterprise Aggregate Switches)
  5. Mike Fratto (2011-03-07). "When MLAG Is Good Enough". Network Computing.