vPC and MLAG coordinate separate switches
A device connected to two different switches can use a single logical bundle when the switches coordinate as a pair. Cisco Nexus virtual port channel (vPC) and Arista EOS MLAG provide this behavior. The downstream device sees one port channel, while the peer switches retain separate control planes. A peer link carries coordination information and can also carry data in some cases.
The peer switches must agree on the downstream bundle and its relevant settings. Each implementation has its own rules for keeping both peers from forwarding independently after a loss of peer connectivity. The single bundle visible to the downstream device does not establish interoperability between different vendors’ peer protocols.
StackWise and VSS present one logical switch
Cisco StackWise or StackWise Virtual and Virtual Switching System (VSS) combine members into one logical switch. A cross-member bundle can then use ports on different physical members of that switch. The stack is managed as a logical system. In a vPC or MLAG pair, each peer remains a separate switch that coordinates the shared bundles with its partner.
Try it in a lab
- Connect a test device to a supported switch pair with one bundle member on each peer. Confirm that the test device reports one bundle and that each peer has its local member and an operational peer relationship.
- Take one downstream member link out of service in the lab and verify that the bundle can still pass traffic through the other peer. Compare how you inspect the pair with how you inspect a cross-member bundle on a switch stack.