From: ccie2be (ccie2be@nyc.rr.com)
Date: Tue Mar 29 2005 - 11:45:48 GMT-3
Hi,
Assuming I correctly understand your topology, there are a couple of
problems you need to deal with.
1. On R1, the rip routes (AD 120) that are redist into ospf (AD 110) are
being advertised back to R1 by R2. Because the AD of ospf is lower than the
AD of rip, R1 will prefer the path to the rip routes learned from R2. this
is known as route feedback. I know of 2 ways to prevent this problem:
Lower the AD of rip to below the AD of ospf 110 with the distance command or
use the distribute-list command to filter the rip routes on the link between
R1 and R2.
2. The routing loop with R3 and R4. Starting with R3, when it redist the
eigrp routes into ospf, R4 will now have 2 paths to those eigrp routes - the
1st path via it's eigrp interface (a native path) and a 2nd path via R3.
The same is true if you were to start with R4. This isn't a problem because
the AD of eigrp is lower than the AD of ospf, so both R3 and R4 will always
prefer the native path to the eigrp routes over the path via ospf. This
means nothing special has to be done as far as the redist of eigrp routes
into ospf is concerned.
Now, consider what happens when ospf routes are redist into eigrp. Starting
at R3, when the ospf routes are redist into eigrp, and then advertised to
R4, R4 will have 2 paths to these ospf routes - the 1st path via ospf
natively and a 2nd path via R3 through the eigrp network domain. However,
when any routes are redist into eigrp, by default, these external (to eigrp)
routes are given an AD of 170. So, even though R4 will have 2 paths to ospf
routes, it will prefer the path with the lower AD which are the native ospf
paths. So, again, nothing special needs to be configured.
That said, when R4 redist eigrp routes into ospf, the redist routes will
include, by default, both the native eigrp routes and the ospf routes which
were redist into eigrp at R3. R4 then advertises all these routes into ospf
which means R3 will learn of these ospf routes natively via ospf and via R4
as external routes. Again, there's no problem because R3 will always prefer
internal ospf routes over external routes to the same destination.
But, in this scenario, what benefit is it to advertise external eigrp routes
(these routes were originally ospf routes) back into ospf? Of course,
there's no benefit. So, what you can do, but don't have to, is configure
both R3 and R4 to only redist eigrp internal routes into ospf. You would
use a route-map and match route-type internal to do this.
The same option of only redist internal ospf routes also exist and you don't
even need to configure a roui-map to do this. You could just do this:
router eigrp 1
redist ospf 1 match internal subnets
HTH, Tim
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
zahid
Sent: Monday, March 28, 2005 7:26 PM
To: ccielab@groupstudy.com
Subject: redistribution----> Routing Loop
Hi Guys,
I need your help I am facing small problem in redistribution of RIP-OSPF
-EIGRP-OSPF.
RIP-e0 (R1)-e1-ospf A39----e1(R2)
/ \
/ FR\
/ A0 \
(R3) (R4)
|_EIGRP_|
R1 running RIP on e0 and OSPF on e1 (Mutual redistribution)
R2 running OSPF area 0 on FR and area 39 on Ethernet (R2 have multipoint
sub-interface for FR)
R3 running OSPF on FR and EIGRP on Ethernet. (Mutual redistribution) with
out route-Map
R4 running OSPF on FR and EIGRP on Ethernet. (Mutual redistribution) with
out route-Map
when IGP complete the loop is coming into the picture, which
roots(prefix) was redistributed from the RIP into OSPF , R1 is learning
these prefixes from the R2 Ethernet and on R3 these prefixes is coming
from R4 FR interface.
What are the methods to solve this problem?
I tried a lot but hard luck.
Kindest Regards
Muhammad.
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