From: Jonathan V Hays (jhays@xxxxxxxx)
Date: Sun Jul 21 2002 - 14:23:24 GMT-3
Hi Tom,
Thanks for sharing your experience. I am a bit confused on a couple of
points in your post. Maybe you or someone else can answer.
1. Was it enabling or disabling split horizon that solved your problem?
I understand that split horizon's basic function is to NOT send network
advertisements out the same interface it came from.
You say below the problem was a router "that had not had split horizon
enabled on the physical interface" - implying that you ENABLED split
horizon to fix the problem. This makes intuitive sense, as enabling
split horizon means fewer updates. But your first "lesson learned" below
says to DISABLE split horizon on physical FR interfaces. I believe that
split horizon is disabled by default on frame-relay interfaces, correct?
Can you clarify this?
2. Could you give a more detailed explanation of the mechanism of the
problem and its solution?
Thanks,
Jonathan
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Tom Larus
Sent: Sunday, July 21, 2002 9:47 AM
To: ccielab@groupstudy.com
Subject: Split horizon practice lab experience to share
Another example of how the basic stuff is the most important stuff.
Here I was working through my materials from NMC-1, to learn the lessons
that I may have missed during class and reinforce the lessons I did
learn. My ISDN link (ospf demand circuit) was coming up more than it
should, but not so often that it was an ISDN config problem.
I figured out that it was instability in the routing tables. Debug
output showed a route from the ospf domain being advertised from the
igrp domain, and the problem turned out to be a router at the far end of
the igrp domain that had not had split horizon enabled on the physical
interface.
Lessons learned:
1) Disable split on physical FR interfaces on which DV protocols are
running (basic point right out of Caslow/Pavlichenko). Missing this
simple step could cost a lot of troubleshooting time if it were to
happen in an exam setting.
2) If you see routes coming over from an IGRP or RIP router that should
not be coming from that router, think of split horizon. Do not go crazy
wondering if it is a quirk about classful routing protocols. Just think
split horizon. (Think of quirks about classful routing protocols when
you are having trouble getting a route advertised to or from an IGRP or
RIP
router)
3) When you are having problems keeping an ISDN link quiet, do not
instantly assume that you have misconfigured the ISDN link, that you
need to put no peer neighbor route on another router, or that you need
to use a distribute list of some sort (you may need one of these, but
don't assume
so.) The problem may simply be instability in your IGP tables.
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