MLAG can be used at various places in the network to eliminate bottlenecks and provide resiliency, including the spine layer and leaf layer. But the most common use case for M-LAG is using this feature at the Leaf layer. At the leaf of the network, servers with multiple interfaces connect to the switches using NIC bonding, or standard Link Aggregation. With two of your leaf switches in an MLAG pair, you can now have active/active redundant connections from the server to the switches.
M-LAG negotiation packets are sent over peer link and determines the master and slave M-LAG peer. From the traffic forwarding perspective, master and slave state is not important, both of the peer will go on to forward any traffic. It will depend on how server are sending the traffic. If server uses both of the link, both the master and slave will forward the traffic. The other duty of peer link exchanging traffic state information like MAC address entries or ARP entries.
Every link in M-LAG domain is in up state. All traffic will be forwarded by all members of M-LAG domain. According to topology, yellow and red traffic are successfully forwarded. Uplink interface of the both members are also used as a hear-beat between 2 endpoints.
If the peer link goes down, Slave device changes its interface status to error down state and traffic is only forwarded over master device.
Yellow and red traffic is sent over master device until the peer-link is recovered. After peer-link is recovered, slave device waits for some time and restore the other interfaces.
Here are some other protection scenarios;
If master device goes down, or the link between master device and server goes down all traffic will be forwarded over the slave one.
If the uplink interface of master device is down, both master and slave will be active. But the peer link will be used to transmit data which is coming from server to the master node. But in this case I assumed that we are sending our heart beat packets as Ethernet frame. If hear beats can’t be received by slave, peer-link will not be used to transmit data.
