Networks of today demands lot of redundancy, be it link level redundancy or device level. Service providers employ various methods to provide this level of service for their customers and one of way is Link Aggregation Group (LAG) where multiple Ethernet links are combined to form a single aggregated group, thereby increasing bandwidth and providing redundancy. 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 uses a control protocol called LACP for its operation.

Instead of having multiple link i.e 2, 4 or 8 aggregated in one Lag, vendors suggested a proprietary solution to have the device level redundancy built as well in Lag and result was Multi-chassis Lag (MC-LAG). As this is proprietary solution, all the vendors do support it but with little difference from one another. Cisco calls this solution as Multichassis Ether Channel, Alcatel calls its MC-LAG.

The CE (Customer Edge) device is completely unaware that its Ethernet links that belong to the same LAG (Etherchannel in Cisco) are connected to two separate PE (Provider Edge) devices. We can assume PE device here as Alcatel 7750. The two PE routers each have one LAG connected to the same CE device. At a time, only one PE router’s LAG ports are active and carrying traffic. The other PE router’s LAG ports are standby and only become active when failure is detected in the active links. The PE routers perform election to decide the active and standby router.

If you see the above figure, from CE’s perspective, all 4 ports belonging to a LAG are connected to a single service provider device (here PEs). All 4 ports are active, but only 2 ports are UP at a time; the other 2 ports are in DOWN state. On the both PE routers, as before we have to create a regular LAG towards the CE device and on top of that we configure MC-LAG separately where we define MC-LAG peer (i.e. 2^nd PE address) and LACP parameters. The MC-LAG control protocol information is exchanged between PE routers. This exchange results in active/standby selection, and ensures only one PE router’s ports are active and carrying traffic.  MC-LAG control protocol runs only between MC-LAG peers. Both PE routers send an exactly same {Admin Key, System ID, System Priority} information in the LACP packets.

Link Aggregation Control Protocol (LACP) detects multiple links available between two devices and configures them to use as an aggregate bandwidth. The two sides detect the availability of the other side by sending LACP packets. One end is an Actor, while the other end is the Partner. During LACP negotiation, (Admin Key, System ID, System Priority) identifies the LAG instance. So, for a LAG, all participating ports on that device must have the same values for these 3 fields.

Regards

Mohit Mittal