Go to the section on OSPF or RIP, and you will certainly find excellent
whitepapers covering issues with redistribution. As far as "the guts of
redistribution", you are right in that it depends on the routing protocol.
But if you remember this simple concept, it's not that bad: Redistributed
routes or networks behave as though it was natively originated by the
redistributing router. That is, OSPF will insert type-3 or type-5 LSAs
depending on whether the redistributing router was in area 0. RIP/IGRP will,
assuming that they have classful masks, simply broadcast the routing table
entry to their non-passive interfaces.
Good luck on your exam. If my notes help you in any way, please give me
feedback after the exam. I am due in August 12-13.
Daniel C. Young
Sr. Network Engineer
CCNP (ATM, Security & Voice Specialist),
CCDP, CCSE, MCSE+I
SBC Internet Data Center
(949) 221-1928 Work
(714) 350-8945 Cell
From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx]On Behalf Of Jim
Sent: Friday, May 25, 2001 7:42 AM
Subject: Another redistribution/connected question
I've been playing with redistribution some more, and I've run into some
behavior I can't quite explain. I've noticed that sometimes when
redistributing, a connected network won't make it in with the rest of the
routing protocol's domain; sometimes it does.
For example: Suppose we have:
R2 does redistribution between OSPF and IGRP.
Sometimes when I've done this, the connected network (10.2.3.0 in this
example) goes into the ospf domain without anything else needing to be
done. Sometimes, though, I need to redistribute it into the OSPF domain
explicitly. I haven't been able to play with this quite enough to know
what makes the difference. Does anyone know? Is it related to which
protocol is used (IGRP vs. RIP, say)? Does it have to do with whether
redistribution is controlled with route-maps? I plan to test these two
theories if I have time.
Another question: in scenerios like I describe above where the routing
domains look like stubs off of each other (i.e., there's only one
redistribution point between two protocols; it looks like a tree), is
split-horizon enough to prevent route feedback? I used to put in
route-maps to control redistribution as a matter of habit, but now I'm
concerned about having them counted as "extra" commands (although I can
also give a pretty good "belt and suspenders" speech if needed). More
recently, I've just used split-horizon to prevent route feedback. That
seems to be working, but I might also be missing something.
Finally, does anyone have a good, really in-depth resource on
redistribution? I've read Doyle v1, but I'm looking for something that
goes into the guts of how redistribution works. For starters, I'm still
not clear on exactly where the redistributed routes come from. Are they
routes that are in the routing table? Are they routes that are in the
database? Or does it depend on the routing protocol -- OSPF takes the
routes straight from the database, while IGRP or RIP have no database, so
they have to take routes straight from the routing table (which would also
explain my confusion about connected routes)?
Lots of questions, I know. I have less than a week before my test, and I'm
dissecting redistribution. :)