Subject: [armd] 3 tier networks ...

From: Kireeti Kompella <[email-masked]>
Date: Tue, 15 Nov 2011 18:00:13 -0800

To follow up my comments yesterday, I think I understand somewhat better why the 3 tier network "fits most scenarios" (as someone commented at the mike) even if those scenarios are not really 3 tier.  In fact, the so-called 3 tier network omits the hosts, which add one (sans virtualization) or two (vSwitch + VMs) more tiers that are of some slight interest to ARMD.

The key appears to be that the 3 tier network captures the L2-L3 split that is all so crucial to the ARMD discussion.  Forgive me if this is obvious to all, or has been done to death already on this list.  The "PE-P" split doesn't quite capture this.  The L2-L3 split is better captured by three tiers: VM/host; L2 up to L2-L3 boundary; and L3.  The L2 bit can be as big as the entire network, and as small as two entities (VM/host + access switch).  Correspondingly, the L3 bit can be as small as nothing, or as big as pretty much the entire DC network.

What that says to me is that the canonical picture ("Generalized 3 Layer Data Center Network Architecture" or "typical topology"), from ARMD's pov perhaps should delineate this split, rather than the physical topology.  The picture in my head (which I am reluctant to depict in an ASCII diagram; attached is a PDF) is an L3 "core" bubble, surrounded by a number of L2 bubbles, each divided into what Benson referred to as "resolution domains" (instantiated today most often as VLANs).  This level of abstraction hopefully focuses on ARMD's central problem, and doesn't get hung up in details that don't matter as much.

Kireeti.