How to connect DC core to Campus core?

[Alek5942]

Hi, my question is about connecting DC core to Campus core. For example, there is DC core which consists of pair of Nexus Switches, and Campus core which consists of pair of stacked Catalyst switches how should they be connected to each other, should be L3 connectivity between them or L2 connectivity with spanning vlans between them and etc.?

[MHM Cisco World]

MHM

[Alek5942]

I didn't get this part "or Routing
usually the NSK vPC run HSRP so there is L2 between NSK vPC and switch stack
the client connect to stack SW is use VIP HSRP (in NSK) as GW. " Can you please clarify? And attach some diagram for that

[MHM Cisco World]

MHM

[Alek5942]

@MHM Cisco World  but according to your diagram, it looks like Nexus is Core not only for DC, but also for Campus. I'm asking how to connect two cores. For example, there is location where there are users and Data Center, do we need to interconnect Campus Core (catalyst switches) and DC Core (nexus switches).

[MHM Cisco World]

draw topolgy to get how your network look like 

MHM

[MHM Cisco World]

MHM

[Joseph W. Doherty]

All true but not a question about L2 within a DC or interconnecting DCs.  Believe question is about connecting, same site campus (users) core to DC core.

Assuming both campus core and DC core are L3 capable, OP request is whether they should interconnect L2 or L3.  Again, generally if either L2 or L3 will work, the recommendation is to use L3.

[Joseph W. Doherty]

Normally I would recommend L3 between them. Modern network designs tend to avoid spanning L2 across multiple devices, for multiple good reasons.

[Alek5942]

@Joseph W. Doherty  thank you for the reply. Example of the setup is: there is location where there are users and also Data Center, and we need to interconnect Campus Core (catalyst switches) and DC Core (nexus switches). So, according to you, the best choose would be to use L3 interconnection, correct? Did you have any experience where L2 would be better choose for such case?

[Joseph W. Doherty · Accepted solution]

"So, according to you, the best choose would be to use L3 interconnection, correct?"

Yes, for pretty much the same reasons that a campus core and distribution connections are usually L3 or connections between multiple campus cores are usually L3 (or even why modern network designs suggest distribution and access connections be L3).

"Did you have any experience where L2 would be better choose for such case?"

Personally, no, but likely there's some situations where it might be.

Approach cautiously such a "need".  You don't, for example, want something like a broadcast flood to take down your core.  Nor, do you want something like STP's relatively slow convergence to pause core operation.  Nor, do you want core multicast issues (some addressed by Cisco's PIM snooping).

As you mention using Nexus in your DC, are you using FEXs?  If so, consider the logical L2 and L3 topologies, and why they are.

Lastly, on L3 switches, you have practically the same performance L2 or L3, so why would you use L2 between the campus core and DC core?

Possibly are you considering port-channel on Catalyst stack to vPC on Nexus pair?

[Alek5942]

@Joseph W. Doherty  Thank you for the detailed answer which really makes sense. The thing is, I see such design in our lab with L2 between DC core and Campus Core's switches and I also think that it's wrong design and I'm considering to change it. So, I'm just trying to understand, if there is any valid reason to have L2 connection between Nexus (DC core) and Catalyst (Campus core). I think it's just a bad design, so there is no solid, justified reason for that. Could you please clarify a couple of your statements:

"As you mention using Nexus in your DC, are you using FEXs? If so, consider the logical L2 and L3 topologies, and why they are." - I didn't understand what you meant here.

"Possibly are you considering port-channel on Catalyst stack to vPC on Nexus pair?" - Yes, but only in the case if it's L2 connectivity between Catalyst & Nexus. Because if there is L3 connectivity between them, vPC and portchannel won't work, am I right?

[Joseph W. Doherty]

My Nexus vPC reference is how that architecture avoids spanning L2, logically, across multiple devices.  I.e. compare it to pre Nexus DC architectures.

"Because if there is L3 connectivity between them, vPC and portchannel won't work, am I right?"

Don't recall.  Assuming it does work, what would be the advantage?  Possibly faster dealing with link up/down.  Possibly reduced routing topology.

But is it extending DC VLANs to campus core?

If it does work, it would be like settling up a L2 port channel but routing across it using SVIs or not.

As to lab design being bad, since you can run L3 over, really need to see what's actually being proposed.

Who designed the L2 approach?  Can they not explain why?

[MHM Cisco World]

Change L2 to L3 ? As a friend Dont do that' 

Read more about why L2 need in DC' I think you totally lost.

Goodluck 

MHM

[Joseph W. Doherty]

@MHM Cisco World, again, sure L2 needed in DC, but would like a reference why it should be used to connect to a network external to the DC itself.