From: Tom Larus (tlarus@xxxxxxx)
Date: Sun Jul 21 2002 - 13:47:26 GMT-3
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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