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

[Paul_Russo]

This has been a great discussion. I just want to make a few points.

From the edge device's point of view it has a LAG that it will hash the traffic out to the core the same way whether it uses MLAG or stack. To it there is no difference and if a link or MLAG switch fails it will move it's connections no differently than if it was going to a stack.

The ISC link is only used for redundancy as Daniel drew or for any device that is not dual homed into both switches. If the core is done correctly there should be little traffic going across this link.

Lastly in order to get the load distribution across both cores in a L3 environment you need to have the two MLAG switches as Active/ Active. This is explained in the concepts guide. Today this is done with having a ACL on the ISC links that blocks the VRRP updates. Going forward, may be in 15.7 can't remember, you will be able to do this without an ACL

In my opinion if you are looking for redundancy in the network you really should be using MLAG. The cost and equipment is the same, assuming you are not using stacking ports. What I mean by that is that if you use 460s or 670s or any other switch type and use native 10G/40G ports to connect them together for MLAG your cost is the same for stacking. The ports are either set as regular 10G ports or stacking ports.

Hope that helps
P

[Paul_Russo]

Hey Ed yes. and if you are not in active/active then all of the routed traffic going to the slave will go across the ISC>

P

[Ed_McGuigan1]

You've got me thinking now Paul. We have a layer 3 environment and we don't have VRRP blocked across the ISC as best as I remember. So we are talking about both routers believing they are the Master and that not being problematic.

Will need to read the concepts guide.

[Ed_McGuigan1]

I think the point with the first diagram is that the two switches function as one logical switch. Traffic flows from A along one of the MLAG ports into one side of this logical switch. That traffic must now flow to B and the logical switch sees that it has a LAG going to B. It takes no account of the fact that one port in the LAG is on the local slot and the other on the other slot and just splits the traffic. This results in traffic transiting the stacking links that are viewed as the "backplane" of the logical switch.

[Stephane_Grosj1]

Hi,

why are you saying that? The end-systems are using regular LAGs, and load-sharing happens normally. If you look at the middle picture from Daniel's post, A has a LAG and traffic is flowing through it on both links.