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From: aaron@dragoon.telcom.arizona.edu (Aaron Leonard)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: poor inter-area routed DECnet performance (long)
Message-ID: <21978@megaron.cs.arizona.edu>
Date: 9 Jun 90 02:05:04 GMT
References: <21919@megaron.cs.arizona.edu> <25934.266f66c4@ccavax.camb.com>
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Reply-To: Leonard@Arizona.EDU
Organization: Univ of Ariz Telecom
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In article <25934.266f66c4@ccavax.camb.com>, tinkelman@ccavax.camb.com 
(Bob Tinkelman) corrects an assumption I made in my earlier posting
concerning poor inter-area routing performance.  I had implied that
traffic between two endnodes in different areas on the same Ethernet
will pass between the area routers.

|> >             Note that the lev II routers are NOT the bottleneck;
|> > traffic that flows between e.g. CIRRUS and UAZHE4, which passes
|> > thru MAGGIE and UAZHE0, but not thru PANCHO, is quick.  
|> 

Bob:
|> My comment is that DECnet will use the intermediate level II routers only
|> to help the two end nodes find each other.  Once the circuit between them
|> is established, CIRRUS and UAZHE4 will communicate directly with each other.
|> This means there will be no intermediate DECnet routing *and* max size 
|> Ethernet packets can be used.
|> -- 

Excellent!  We're on to something here.  Bob is right - the DECnet traffic
between CIRRUS and UAZHE4 will indeed short-circuit the area routers and
travel directly over the ethernet. 

And this result, in fact explains why the performance is so poor when the
cisco router is entered into the loop: because the short-circuiting of
same-Ethernet circuits between different-area nodes ONLY HAPPENS when the
nodes are END NODES!  When the set-up path is: Area-A-endnode-on-Eth1 ->
Area-A-lvl-I-rtr -> Area-A-lvl-II-rtr-on-Eth2 -> Area-B-lvl-II-rtr-on-Eth2 ->
Area-B-lvl-II-endnode-on-Eth2, the short circuit between "Area-A-lvl-I-rtr" and
"Area-B-lvl-II-endnode" is never made.  Rather, all traffic between the
endnodes will continue to flow thru every single router in the loop.
(Oh, for an icmp redirect!)

I verified that this is not a problem with the cisco; rather, an identical
path traced thru a VAX/VMS router produced identical results - so by
definition, the cisco is routing correctly!

This brings up, then, another question: if all my DECnet connections
into my ciscos are to direct-attached ethernets, then is there any
reason at all to run DECnet routing on the ciscos?  (Assume here that
equal-cost path splitting is not a topological possibility.)  Why not
just bridge the  whole ball of wax?  In this (admittedly pathological) 
case, at least, bridging will produce better throughput and reduce
host load, right?