Re: EGP madness


J. Noel Chiappa (JNC@XX.LCS.MIT.EDU)
Fri 14 Nov 86 19:19:15-EST


        Hi. This is a canned message. My apologies for not sending a
personalized reply, but this question gets asked once a month and I got
tired of typing the same message. (An MIT hacker defined 'Hell' as
'answering the same bug report over and over again'.) It was answered most
recently on:

Fri 3 Oct 86 01:48:28-EDT
Fri 31 Oct 86 16:03:42-EST

        You have asked a question about the infamous 'extra hop
problem'. The problem is not caused by EGP, which is telling you
exactly what the gateway you are a neighbour with is doing itself with
packets to given destinations, but the routing protocol (GGP) which is
used by the core gateways among themselves. It predates EGP, was not
designed with the pattern of information flows that you see in EGP in
mind, and is the cause of the problem.
        As a brief example of the problem, if MIT has core gateway A as
an EGP peer, and Berkeley has a peer core gateway B, then there is no
way (using GGP) for A to inform B that to get to MIT it can go direct;
both B and all its clients (e.g. Berkeley) think they have to go
through A. This is the cause of the funny routes to places you ought to
be able to get to directly, etc.
        Your gateway is just fine, and it's not EGP's fault either. The
extra hop problem will only be solved when GGP is retired; i.e. when the
PDP11 core gateways are replaced by Butterflys, probably. When GGP is
replaced the problem will magically disappear without any changes to
EGP.

        For a more detailed explanation of the problem, look in the
TCP-IP archive for a message I sent out at Thu 6 Mar 86 18:16:01-EST
which goes into great detail. (No, I do not know how to access the
TCP-IP archives, so don't send me mail asking for it; I'll ignore your
message. If someone from the NIC sends me the appropriate info, I'll
happily insert it here.) Just out of interest, were you on TCP-IP
then?

                Noel
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