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]

Ok, thanks. Now I am wondering indeed what would be the use of using MLAG here. As Tomasz claims, there's no real benefit here. What would you do? Use MLAG for those ISLs or simply treat them as single links and let STP handle them? Cross-links are not possible right now.

[Stephane_Grosj1]

The benefits are all links active and no extra protocol running.

[Tomasz]

IMO single STP domain makes you benefit less as the link is blocked. MSTP might cover this issue, but more to configure. If those cross-links are not possible just for some time, perhaps you might leave it as is until it becomes full-blown MLAG (then there'll be not much to reconfigure). 

[Erik_Auerswald]

Hi jeronimo,

the topology is valid. No link needs to be blocked with MLAG while STP, EAPS, or similar would need to block at least one (logical) link. You might want to consider adding cross-links (upper left to lower right, lower left to upper right) if possible to optimize redundancy and fail-over speed.

Please see the GTAC Knowledge site for additional info:

• Explaining Two-tier MLAG design
• Key Design Points of an EXOS Multi-LAG
• How to configure MLAG in Extreme switches

Thanks,
Erik

[Tomasz]

Hi jeronimo,

I've encountered this when visiting one of the customers.
From MLAG perspective, it will forward the traffic, but IMO you don't benefit MLAG here. It's like having failover scenario due to diagonal links (A1-B2, A2-B1) outage.
I'm not sure right now if the ISC blocking filters be disabled due to this.

With such four switches you can also think of different non-port-blocking mechanism, it could be EAPS or A and B stacks + LACP... Or perhaps you would like to play with MSTP and manipulating the bridge priorities for every MST instance so for each VLAN root bridge is different one, so all links are used. But that sounds like a lot to do. 😉
All depends on place in the network and your goals and constraints.

Kind regards,
Tomasz