Path: utzoo!utgpu!water!watmath!clyde!rutgers!umd5!hans
From: hans@umd5.umd.edu (Hans Breitenlohner)
Newsgroups: comp.sys.proteon
Subject: Re: Split Horizon vs. Static Routes
Message-ID: <2284@umd5.umd.edu>
Date: 12 Feb 88 17:25:31 GMT
References: <8802091626.AA04046@monk.proteon.com>
Reply-To: hans@umd5 (Hans Breitenlohner)
Organization: University of Maryland, College Park
Lines: 40


I find it unfortunate that there seems to be such overwhelming consensus
(at least among the readers which post articles) that split horizon 
should not apply to static routes.  I would like to present two arguments
against that.

First a very general one.  The configuration in question always seem to
involve an 'othergw' which does not speak RIP, hosts which listen to RIP,
and a p4200, on an ethernet.  The argument further involves something of
the flavor "I don't want to install static routes on all hosts".  Well, 
with all due respect, you have a mismatched batch of equipment, and you
should pay the price for that.  This could be done by installing static
routes, default routes, or by adopting Mike Curtis' proposal (at a slight
performance hit).  I can understand that you would like for Proteon to 
solve your problem, but I think that it is unreasonable to expect it.

The second argument is more specific, and involves a counter-example.
This is not contrived, but actually happened.
I won't mention any names, to protect the innocent (and
the not so innocent).
Take a regional network, consisting mainly of p4200s connected via serial
lines.  Several sites have connections either to Arpanet or to Milnet.  
Many of these also advertise default.  A site with a Milnet connection
decides that its Arpanet traffic should not be routed through its Milnet
connection, and puts in a static route pointing to the rest of the network
for net 10.  Unfortunately this site runs 7.3, and the route for net 10
gets advertised to the rest of the network.  There it overrides the 
best way known to reach net 10 (via default route).
Instant routing loop, instant black hole for net 10, instant disaster!
In some cases it may be possible to manipulate hop counts to avoid this,
but in the case where the network for which
the static route is installed is handled by default instead of explicit
advertisments there is no other way to accomplish the same thing.

In conclusion, there are a number of different scenarios in which static
routes are useful.  In some cases ignoring split horizon might be useful
or expedient, but in other cases it can lead to disaster.  I have no
problem with Proteon adding the flexibility to allow users to ignore
split horizon for static routes, but I think it would be a serious mistake
to do so as the general case.