Proper sizing of VSS virtual switch link

[mcox-cisco]

I am considering using VSS in my collapsed core/distribution on a pair of 6509/Sup2T switches with 6904-40G-2T modules. My closets are 4506/Sup7E, and I have one high-density, mission critical IDF with two fully populated 4510R+E/Sup7E switches. The interlinks will be either 2x or 4x10GE depending on customer decisions to be made. I am trying to find information that would suggest best practice for sizing the VSL. From what I can find, it would appear that not a significant amount of traffic goes across this link if everything in the environment is equally connected to both members of the VSS pairs. So, 2x10GE would be just as good as 4x, or even 2x40GE. However, I haven't found anything definitive yet. Is there a rule of thumb for sizing this link? Thanks for any guidance.

[Reza Sharifi]

All the VSS scenarios I have deployed have been with 2x 10Gig interfaces.  With this config, you should not have any issue and as you noted, there isn't going to be that much traffic cross these links anyway.

HTH
 

[sean_evershed]

If the VSS switches are going to be there for a couple of years then an alternative is to buy 2 x 40Gbps now in order to future proof your capacity. It is my experience that is easier to buy all kit upfront. I find it to harder to get budget approval to purchase upgrades later. Management always want proof that upgrades are required.

[Joseph W. Doherty]

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Posting

The VSL can be the Achilles Heel of a VSS setup.

As you note, if everything is configured optimally, very little traffic should cross this link.

However, suppose a line card fails.  As the sup2t supports 80Gbps slot, in theory, that 80Gbps might need to transit the VSL.  (NB: Hopefully, VSL also wasn't on the failed card.)

So, to size your VSL, consider bandwidth needs if a line card fails.

Also keep in mind, VSL (at least on 6500s) doesn't allow custom QoS, so you also lose some control on traffic management if VSL congests.