MLAG vs Stack what am i missing

[Chris_Chance]

Can't seem to wrap my head around the reason for using MLAG vs Stacking, I'm planning to use 2 x670's as a 10gigabit aggregation hub for our remote sites, but i want redundancy, so the idea was run 1 fiber to each of the x670's and run mlag so if 1 x670 fails, tada still up and working... But then i got to thinking if i stack those 2 x670's and use a standard lag group from 1:1 and 2:1 to the remote site, isn't it IDENTICAL, but i get the benefit of not dealing with the mlag, not having to deal with managing 2 core switches, and still keep the same load balancing, same redundancy, same resilience and high availability? I feel like theirs got to be something here I'm missing

[Mrxlazuardin]

Hi Daniel,

Do you mean that MLAG cannot be used for load sharing like standard LAG?

Best regards,

[dflouret]

Stephane mentioned one important difference between MLAG and stacking:

"MLAG offers a natural local switching compared to Stacking, ie the ISC should be used for unicast traffic only for failover of links (if dual-homing only), while Stacking may use more intensively the stacking links."

Let me translate his words into drawing...

You have switches C1 and C2 stacked together. The stack connects to switch A through LAG1 and to switch B through LAG2.

Traffic flowing from switch A to switch B should (hopefully) be equally balanced amongst all links in LAG1 and LAG2.

We should expect something like this:

Some (half?) of the flows would traverse the stacking links.

If you use MLAG instead of stacking you would see this traffic pattern:

The ISC has filters that prevent unicast traffic from traversing it, except when there's a link failure.

If the link between C1 and B happened to fail, this is what you would see:

BTW, in case you were wondering, this is also the traffic pattern you'd see if you were using stacking and had the same link failure...

[Drew_C]

I wanted to mention that this is great topic for discussion. Hopefully others will pop in and contribute!

[Ed_McGuigan1]

I have wondered the same about my location where the decision has generally been to run two separate systems and MLAG to them.

In actual fact, we seem to have gotten ourselves in a mess by not carefully duplicating config on our parallel routers and there is a need to be very committed to this aspect if one chooses MLAG over stacking. I have also seen upgrading of stacks be problematic so it would be scary to have one stack and hit problems with an upgrade.

[eyeV]

When we have chosen between MLAG and stack, we done lots of tests. We've chosen an MLAG for some reason, one of them is more efficient redundancy between two switches. It's a bit hard to explain, because of my poor English, but I'm going to try.

We use two x670 + VIM4-40G4X-1. These boxes are geographically separated. Then we use stack we combine two 40G ports to one logical stack port, so we have 2 stack ports.

So... if one 40G ports goes down, S1 for instance, all StackPort1 goes down. But if we use MLAG, we still have three working ports. Pretty bad explanation, guys... I've tried...