Re: OSPF & EIGRP redistribution

Hi Scott,

Just to make sure... R1 & R2 : OSPF and R2 & R3 : EIGRP...

Can you confirm you used the 'subnets' in the redistribute statement?

Thanks,

Sent from my BlackBerry. wireless device

-----Original Message-----
From: Scott Strobeck <scott_at_strobeck.net>
Sender: nobody_at_groupstudy.com
Date: Tue, 31 Jan 2012 14:07:18
To: ccielab<ccielab_at_groupstudy.com>
Reply-To: Scott Strobeck <scott_at_strobeck.net>
Subject: OSPF & EIGRP redistribution

Hi everyone,

I noticed today in my lab that I had an unexpected route. After chasing
it down for a while, I found it seems to be an anomaly when
redistributing ospf->eigrp.

Consider a simple R1--R2--R3 lab where OSPF is running between R2&R3,
and EIGRP between R2&R3. R1 & R2 have a loopback advertised into the
OSPF with network statements. Full redistribution between OSPF & EIGRP
is done on R2.

R2's loopback, left at default, will have a network type of LOOPBACK and
will show up in OSPF as a /32. However, in the EIGRP domain (on R3),
this external route will show up as a /24.

Is this expected behavior? Why would EIGRP have it as a /24 when OSPF,
where it came from, has it as a /32. R1's loopback shows up as a /32 on
R3, as expected, so why should this be different for R2's loopback?

This may not seem like such a problem until you take R4 and connect it
to both the OSPF and EIGRP domains, and perform full redistribution,
again. Now, the /24 route from R2's loopback will get redistributed
into OSPF as an external route since there's not an internal equivalent
to cancel it out. Ultimately, the route will get advertised back to
R2.

If you add a 3rd point of redistribution between the two IGP's (R5), now
you've created a routing loop for this /24 route and if you shut down
R2, the /24 route created by R2's loopback remains.

I'm tempted to go ahead and open a new ddts for this, but wanted to
check on here, first. There may a good reason why this happens, but I
can't, for the life of me, think of it.

(BTW, as it stands, this would be a great 'gotcha' for the lab. . . a
potential workaround might be to change the loopback's ospf network
type, or to filter off the route in the EIGRP domain.)

Thanks,
Scott

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Received on Tue Jan 31 2012 - 19:25:54 ART