This is a basic question that has come up with different customers a few times and I thought I would write a small blurb to explain this.

Question:  We would like to use the same vCenter  for VMM integration for our 2 or more ACI MultiSite Sites but we keep getting told to go to MultiPod in that case.  Can you please explain if that is indeed the case and why ? MultiSite Orchestrator can do L2 stretch even with BUM (broadcast, unicast and, multicast) traffic forwarding, so why can’t we stretch the VMM domain across sites ?

Answer:  To understand this,  you have to realize that Multisite is not one Fabric.  It’s multiple fabrics each with it’s own cluster of APICs that can be fully or in part be orchestrated by the MSO.   For Multipod, all the different pods still are one fabric managed by one cluster of APICs.

Now,  the whole idea of VMM domain is that the APIC cluster of a fabric exchanges information with vCenter for a particular VMM domain.   In Multipod since it’s one Fabric with one cluster of APICs, you can stretch your VMM Domain across the entire Multipod pods.    You cannot do this with Multisite, because one Site’s APIC is controlling a particular VMM domain.  Another Site’s APIC cannot control the same VMM domain.  In other words VMM Domain belongs to 1 APIC cluster.

Now, to make sure you are clear,  that does not mean that you need to have multiple vCenters.  

A VMM domain is comprised of  a vCenter + a vCenter datacenter (in that vCenter) + the uplinks for that DVS (from each host) attached for that VMM domain (DVS).   If any one of those 3 variables is different it is considered a different VMM domain.  In other words, you should equate a VMM domain with a DVS.

For example:

you could have a vCenter datacenter with 10 esxi hosts and you create a VMM domain (meaning 1 DVS) spanning those 10 esxi hosts .  You could also have separate uplinks from those same hosts (in the same vCenter datacenter and install another VMM domain (in effect another DVS) spanning those same 10 hosts.  In other words you would now have 2 separate VMM domains i.e 2 separate DVS’s on those same 10 hosts.   Each of those VMM domains can be controlled by the same APIC Cluster or 2 separate APIC clusters (meaning 2 different ACI fabrics).  However keep in mind that the uplinks of each VMM domain (DVS) need to go to the  leaves of the Fabric whose APICs are controlling that VMM domain.

Let’s look at another example:

vCenter A,    Data-Center-X,  uplinks  Ux  = a unique VMM domain and will be controlled by one APIC cluster

vCenter A,    Data-Center-Y,  Uplinks  Ux’  = a unique VMM domain and will be controlled by one APIC cluster

vCenter A,     Data-Center-X, uplinks Uy = a unique VMM domain and will be controlled by one APIC cluster

Now in the above example,  even though the vCenters are all the same, there will be 3 distinct VMM domains.  The 3 VMM domains can be controlled by 1 APIC cluster or by 2 or 3 APIC clusters, meaning APIC clusters for different ACI Fabrics.  Each VMM domain’s uplinks need to connect to the Fabric leaves whose APICs are controlling that VMM domain.

The point is that there are 3 VMM domains in the above scenario.   At the end of the day, for Multisite, you will have to have different VMM domains for different sites, not different vCenters. 

The key is 1 VMM domain == 1 APIC cluster (but 1 APIC cluster can have multiple VMM domains).

Also, from vSphere 6.0 onwards you can do inter VMM domain vMotion ( also called “cross vCenter vMotion” )and that becomes particularly easy (directly from the vCenter UI) if you have the same vCenter.  If you had different vCenters, you can still vMotion with PowerCli.    The diagram below might help you visualize this.  In that diagram there are different vCenters on the different sites of the MultiSite Fabric.  However, you could very well have had the same vCenter and that would still work.  The VMM Domains would still be different.   In other words, in vCenter you would have DataCenterA, and dataCenterB.  DataCenterA would have the esxi hosts of Site A where as DataCenterB would have the esxi hosts of Site B. and that would work.  The VMM domains would still be different and Site A Apics would control VMM Domain A and Site B Apics would control VMM Domain B.   However, both APIC clusters would still point to the same vCenter.

I’m hoping this clarifies the above question.