We want to configure 2 Arista switches by mlag and connect 1 port channeled switch to both of arista's. so in this case, if one of our arista fail the other one will respond. now my question here is in this scheme one of the arista block packets from receiving and only one of them is working, right? and if that working unit fails my traffic travel through second arista and second one will be elect as primary ?

in another word my traffic will not travel through both arista and towards my routers?

Thank you.

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 15 '19 at 19:31

The whole point of MLAG / VPC is to have both link active, as in a standard link aggregation. Otherwise there would be little benefit over xSTP (spanning tree) (yes it converges faster but it's much more complex to setup and not interoperable between different brands).

So if properly configured:

  • when all links are up, you got the full bandwidth
  • when a link is down, both switches continue to forward traffic. If needed the switch with the failed link will transfer the frames to the other switch and they will then be sent to the destination host.
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  • so your mean is if i set one of them arista as primary and then one of my link from my downstream switch to arista 1 drop then second arista can not handle it ? in this case only this switch traffic should travel through arista 2 and other switch traffic travel from arista 1, does not it? – Blackmetal Oct 1 '19 at 18:18
  • MLAG has no concept of primary ports. As @JFL mentioned, all links are active. – Teun Vink Oct 1 '19 at 18:57
  • You may want to read arista.com/en/products/multi-chassis-link-aggregation-mlag, where Arista explains their MLAG implementation in (a bit) more detail. – Teun Vink Oct 1 '19 at 18:59
  • @Blackmetal edited to clarify – JFL Oct 1 '19 at 20:15
  • Understood, so all links are active and if any of links fail i will have link failover, am i right? – Blackmetal Oct 2 '19 at 2:40

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