Re: Split horizon practice lab experience to share

From: ccie candidate (ccie1@xxxxxxxxx)
Date: Sun Jul 21 2002 - 15:30:49 GMT-3

   
 hi
there is a document on the CCO concerning a problem similar to this ,where ospf
 is keeping the ISDN up while you use the demand circuit command .
the situation will happen when that router is redistribution point between ospf
 and IGRP , the ISDN link will install /32 address of the far end in the routin
g table .
when the link goes down , the ospf will consider this a database change (becaus
e of the redistribution of IGRP into OSPF ,when the link goes down the /32 addr
ess disappears ) and the OSPF will bring up the link again .

you may need to use no peer neighbor-route command on your isdn interface to so
lve this problem ..

just a thought . 

--

On Sun, 21 Jul 2002 12:47:26
 Tom Larus wrote:
>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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