From: Colin Barber (Colin.Barber@xxxxxxxxxxxxxx)
Date: Sun Jul 21 2002 - 18:20:43 GMT-3
Another possible solution may have been to change the distance of IGRP on
the redistribution router? Why Cisco think it is better for IGRP to be more
believable than OSPF I will never know!
-----Original Message-----
From: Tom Larus [mailto:tlarus@cox.net]
Sent: 21 July 2002 19:04
To: Jonathan V Hays; ccielab@groupstudy.com
Subject: Re: Split horizon practice lab experience to share
You are exactly right. I should have said "enable split horizon on physical
FR interfaces where a DV routing protocol is running.
As for detail about the problem:
The problem was that a route to a loopback interface in the OSPF domain was
going into and out of the routing table on a router that was speaking OSPF
and that had a an ISDN link which was configured to as OSPF demand circuit
in area 0. Since it was area ), I could not simply "stubify" its area a bit
to isolate it from route instability elsewhere in the network, so I had to
solve the underlying problem to get the ISDN link to stay down as it should.
Of course, one would need to solve the underlying routing problem anyway. I
am beginning to suspect that a lot of the problems that people have with
ISDN demand circuit are really problems with instabilities in their routing
table. That is one of the major lessons I take from this.
As for the mechanism of finding the solution, I turned on more debugging
than I needed, but I think it was debug ip ospf lsa-generation that tipped
me off about the exact route that was causing the problem. Seeing a route
local to the OSPF-only router I am working on coming from way the heck over
at the other end of the IGRP domain made the nature of the problem pretty
obvious.
----- Original Message -----
From: "Jonathan V Hays" <jhays@jtan.com>
To: "'Tom Larus'" <tlarus@cox.net>; <ccielab@groupstudy.com>
Sent: Sunday, July 21, 2002 1:23 PM
Subject: RE: Split horizon practice lab experience to share
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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