Re: OSPF & EIGRP redistribution

I just recreated this, as follows:

R1 (fa0/0 -- fa0/0) R2 (fa0/1 -- fa0/1) R3

R1 Fa0/0 - 12.12.12.1/24
R1 Lo0 - 1.1.1.1/24

R2 Fa0/0 - 12.12.12.2/24
R2 Fa0/1 - 23.23.23.2/24
R2 Lo0 - 2.2.2.2/24

R3 Fa0/1 - 23.23.23.3/24
R3 Lo0 - 3.3.3.3/24

OSPF process 1 running on both R1 and R2
R1
router ospf 1
network 12.12.12.1 0.0.0.0 area 0
network 1.1.1.1 0.0.0.0 area 0

R2
router ospf 1
network 12.12.12.2 0.0.0.0 area 0
net 2.2.2.2 0.0.0.0 area 0

router eigrp 100
no au
net 12.12.12.2 0.0.0.0
net 2.2.2.2 0.0.0.0

R3
router eigrp 100
net 23.23.23.3 0.0.0.0
net 3.3.3.3 0.0.0.0

Mutual redistribution is taking place on R2 for OSPF 1 and EIGRP 100

R1 lists 3.3.3.3 as a /24
R1 lists 3.3.3.3 as a /24

R3 lists 1.1.1.1 as a /32
R2 lists 1.1.1.1 as a /32

If I go into R1 interface Lo0 and add the following
ip ospf network point-to-point

R3 now shows 1.1.1.1 as a /24

This is normal behavior... am I missing something??

-Christian Hunter

On Tue, Jan 31, 2012 at 2:07 PM, 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 - 14:27:44 ART