Topology: MLAG

[jeronimo]

Hey,

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?

Thanks.

[jeronimo]

@roy Oh and to answer your question: yes, it does indeed work. It even works with a 3rd party RSTP "ring" attached to two of the four switches

[FredrikB]

I have a large customer that is moving away from EAPS due to several reasons, including issues with maintaining configs (i.e. consistency between switches in the rings), the poor link utilization (one link blocked), licensing and stability reasons. They are moving to an MLAG setup where the core distribution has multiple MLAG pairs and the local distribution (multiple for each core dist pair) consists of switch pairs running as MLAG peers. We asked Extreme TAC about the feasibility of running MLAG to MLAG with only the direct links (not full mesh with the X in the middle) and after some investigation on their side, even involving a meeting with us and some high-ranking TAC experts, they came back and said it would work and (I think officially) be supported.

If you think of it, each MLAG peer pretends to be one single switch presenting a LAG to the other end of the LAG. IF this is the case from one side of an Extreme MLAG pair to a server with dual NICs or if it is between two MLAG pairs shouldn't make a difference. Other vendors support this so it would be strang if Extreme wouldn't. That said, in the core I would still go for full mesh.

Someone said you wouldn't gain anything from an MLAG-MLAG setup if it's not full mesh, but I certainly don't agree. We get 2 x the capacity compared to both EAPS and STP unless we go through a lot of hassle to create multiple domains, and that makes for a very coarse load balancing indeed. The most important gain is that MLAG seems more stable than EAPS and STP, even if there are still scenarios where MLAG can create problems.

B.t.w. if you use VRRP in an MLAG pair, did you know that the fabric-routing allows both peers to route?

https://gtacknowledge.extremenetworks.com/articles/How_To/An-example-of-VRRP-fabric-routing-configuration-to-achieve-active-active-forwarding-routing-on-all-VRRP-routers

Another tip for you is that if you have two fiber pairs between two sites, you can still have four links, and have your full mesh if you like. Just use BiDi SFPs! CWDM is another alternative that is not very expensive today.

Please, people, STP in this day and age???? I always say STP was great in the 80's when it was invented, but today there are almost always better alternatives.

/Fredrik

[Paul_Thornton]

Interesting this question came up - I was about to post about a near identical setup.

The benefit I see of this "back to back" MLAG is that both sides just see a LAG, so you have a true active-active situation for traffic in either direction. The alternative is EAPS or STP; which would work, and you could probably arrange multiple VLANs to get maximum use of both connections. But it is still more work and config compared to the MLAG example.

When I did some testing of this (well actually ended up deploying it with 2x X460G2s on one side, and 2x customer X440s on the other side), I was seeing some destination macs in the same L2 network that just didn't work. It had all the hallmarks of a situation where a LAG without LACP was unidirectional so a packet was hashed and sent down a link that just didn't work.

I can't see why this shouldn't work, and so long as you have LACP enabled (we do all remember to always enable LACP, don't we folks) you have a control protocol to deal with connectivity problems where the link may appear to still be up and avoid the blackhole situation I saw.

The "Two Tier MLAG design" topologically isn't quite the same as what we're describing here - it has the cross connections in place. Are they required to make this legal (I can imagine, given that I had packets going nowhere, that this would fix what I saw) - or should this "just work" with the simple two cables between two sets of switches setup?

Paul.

[Erik_Auerswald]

This two-tier-MLAG construct is used in quite a few EXOS based networks, because all links are active as opposed to blocking links with STP or EAPS.

[jeronimo]

I found using MLAG on both sides between switches quite a thought experiment, since you usually somehow associate MLAG with the "server"/"switch"-side and a port-channel/LAG with the "client"/host-side. In this scenario, MLAG is both...