The MLAG Components

When we want to connect two switches and make it an mlag, the following things should be kept in mind:

  • Both the switches should be advertising the same system id to the partner
  • North bound or south bound BUM traffic should not be sent twice to the LACP partner (some rules possible in Linux to drop the traffic?)
  • MAC table should be synchronized between the two switches
  • MAC Learning should be such that no bouncing of MAC addresses coming from the LACP partner happens
  • Neighbor table should be in sync between the two switches (since it is not really L3, but somewhat L2 too)
  • Since we use LACP, the connection between two switches and the partner must be direct

The links to the switches can be many, asymmetrical and when two pairs of switches form a MLAG – it will be a fully meshed network (all connecting to all) with each pair having the same system MAC address (when a host is there instead of switches, this same system MAC address is taken care of by the bonding driver)

Nice summary here. Apparently, cumulus linux uses “clagd” to periodically synchronize the MAC database to the peer switch. “clagd”, among other things sets up the peering relationship, determines the primary/secondary roles for sending BPDUs etc. There is also a presention by Cumulus on MLAGs which is a simple but nice read


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