[HECnet] Network topology according to MIM.

[Paul_Koning at Dell.com]

> On Jan 15, 2016, at 10:33 AM, Robert Armstrong <bob at jfcl.com> wrote:
> 
>  Sorry, I just don't see the need for hot standbys and redundant paths.  This is HECnet we're talking about - when was the last time anybody used HECnet for any real work?  I have no objection to backup paths and routers, but leave them disabled until they're needed.  But otherwise, let's simplify the network management as much as possible.  

It's fine to use just a basic network topology.  My point was that the topology you objected to is a valid and desirable real world topology (in other settings, arguably).

But note that unless your topology is exactly a tree (no redundant paths at ANY spot), you may have asymmetric paths.  Consider two paths A-B-Q-X and A-C-P-X (with equal circuit costs throughout).  A-X traffic would go A-C-P-X, X-A traffic would go X-Q-B-A.  That's normal.  Asymmetry is not a problem; the DECnet architecture has absolutely no interest in avoiding such a thing.  What it does try to do (and in this it explicitly differs from IP routing algorithms) is "determinism": any given network topology will, in the steady state, behave in a consistent way independent of what sequence of events lead to that topology.  This is, for example, why DECnet has tiebreaker rules in route selection and designated router election that are designed for determinism, unlike, say, the OSPF designated router election that explicitly aims at a different property.  (One can argue forever which choice is better, and I don't intend to get into that now; the point simply is that DECnet chose that principle and applied it consistently, while other designers chose differently.)

	paul