From: Daniel C. Young (danyoung99@xxxxxxxxxxxx)
Date: Sat May 26 2001 - 16:13:32 GMT-3
Jim,
There has been a lot of discussion on this topic matter. Check the archives
month by month with the key word "ospf redistribution", and I'm sure that
you will find something. But I will give a humble attempt to summarize what
I personally have learned.
Connected networks
OSPF will not redistribute connected networks when there is no matching
network command under the router configuration. The key is to check the
masks that you apply on the network statement. OSPF will check which
interfaces (connected networks) will be included by the network statements.
If it falls outside the mask, the you will have to perform a "redistribute
connected" to have OSPF generate a proper LSA for it (type-5 for a non-area
0 router and type-3 for area 0). It has nothing to do with route-maps unless
the route-maps somehow filter the routes. IGRP and RIP are classful distance
vector protocols, so they will only accept routes that have classful masks
(/8, /16, and /24). Also if you have subnets included, like 172.16.1.0/24,
then don't forget to use the parameter "subnet".
Stub network -> split horizon?
No, split horizon is a distance vector protocol feature used to prevent
routing loops. OSPF uses cost to calculate SPF, builds a complete topology,
then inserts only loop-free routes into the IP routing table. I would
suggest that you develop an understanding of when route-maps or
distribute-lists are truly needed. I have a feeling that if you just apply
them blindly and fail to justify your reason when asked by a proctor, you
may lose points on the exam. Start out without any filters, then observe
each route on the table and FOLLOW ITS PATH -- as if they were static
routes. You will develop a very good understanding of routing protocol
behavior when you do this.
Redistribution resources
http://www.cisco.com/pcgi-bin/Support/PSP/index.pl?i=Technologies
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.
Regards,
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
young@pobox.com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of Jim
Graves
Sent: Friday, May 25, 2001 7:42 AM
To: ccielab@groupstudy.com
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:
(OSPF
[10.1.0.0/24]---R1---[10.1.2.0/24])--R2---([10.2.3.0/24]---R3---[10.3.0.0/24
]
IGRP)
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. :)
Thanks all,
Jim
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