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