Re: OSPF & EIGRP redistribution

This is normal behavior. OSPF will see the loopback as a special 'LOOPBACK'
network type, and will advertise the interface as a /32. When you
redistribute OSPF into EIGRP, you're essentially matching any interface
with OSPF enabled, and of course any OSPF routes installed into the RIB.
The /32 route is not actually in your RIB, it is only in your
self-generated Type-1 LSA. This is why EIGRP is advertising that route as a
/24, just as it would if you had done a 'redistribute connected'.

When you set the network type to 'Point-to-Point', OSPF advertises the
interface with its true prefix, and therefore you achieve consistency
between what OSPF is advertising, and what EIGRP is redistributing.

On Tue, Jan 31, 2012 at 11:07 AM, Scott Strobeck <scott_at_strobeck.net> wrote:

> 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 - 11:38:49 ART