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Topology: MLAG

Topology: MLAG

Contributor III

Can you validate that the following is a valid setup using MLAG.

Sites A and B contain two MLAG peers each.

There are two independent links between sites A and B, and in order to connect both sites, both peers can use MLAG at each end. (Each group of switches thinks that it is talking to one switch at the other end.)

This would make the two links active, thus double the bandwidth compared to STP which would always block one ISL. But I'm not sure about STP in this scenario.

I couldn't really validate nor not validate such a setup from reading the manual. Is it valid?

Anything that comes to mind about this?




Contributor II
Hi Paul,

the setup should work without the cross connections, we are using this exact setup in the network of the small company I work for (we are using older EXOS switches, not -G2, but this should still work). We have a switch pair on the office floor and another one in the basement connected via two-tier-MLAG using just two links.


Paul has a point. I was testing back-to-back MLAG topology concerning all kinds of link failures:
* direct: the simplest failure: the port goes physically down
* indirect: 1. there is something in-between that isn't passing data anymore, or 2. there is nothing in-between so something on the other side has died but the link is still up

Now the challenge is of course an indirect link failure. Like when you "pause" the link in GNS3, when the ports stay up but data isn't passing. Whether you implement the cross connections or not, I found that when I pause one of the links, some traffic starts getting blackholed. Probably because in back-to-back MLAG there is nothing like LACP which would watch the links (or loop-protect like in STP which stops using the link if there are no more BPDUs).

New Contributor
@jeronimo Did your "Back-to-Back" design work out? I just asked GTAC, before finding this post, about this same setup and they told me that it wouldn't work.

Contributor III
@roy That's interesting. But first, we should find out what everyone is talking about. Also if "back-to-back" means the same thing as "two-tier" and what exactly that is.

My post was mainly about this: with the subtle difference that I don't have a full mesh but only one ISL each. So we already have two setups. What exactly did you ask from them?

Information in general about this subject seems very sparse, not many people seem to have a full grasp of this technology. I have the feeling before raising false expectations they will just tell you it won't work. In my case I was told by a local engineer during redesign of our previous DC network that this wouldn't be a problem. The full mesh would be recommended, but it would work either way.