Re: OSPF & EIGRP redistribution

This is the expected behaviour. When you redistribute EIGRP into OSPF,
EIGRP will do this in a two step manner. First off it will get all of
routes from the "show ip route ospf" command. Secondly it will then get
all of the connected interfaces (show ip route connected) that OSPF is
enabled on (show ip ospf interface brief). Since OSPF is enabled on the
loopback and it has a /24 mask, EIGRP will bring it in as a /24. The
concept of loopback network type in regards to OSPF has nothing to do
with the local redistribution as redistribution occurs between routes in
the routing table and not from a particular routing protocol's database.

If you remember this two step process that route redistribution goes
through, your preparation will be a lot easier as it holds true for all
routing protocols.

I demonstrate this along with the root cause of routing loops in my
bootcamps. Route redistribution complexity is way overblown and the
reason is people are attacking a problem without knowing the root cause
and the process the IOS goes through to perform redistribution. I'll
venture to bet I could write a blog post on it and keep it about one or
two pages max and 95% of the people who read it will have a full
understanding and never fear route redistribution again. It's one of
the most misunderstood topics in CCIE preparation and really kind of the
most worthless topic as complex route distribution problems are few and
far between these days.

Hope to see some of you all in London this week at Cisco Live!

-- 
Brian Dennis, CCIEx5 #2210 (R&S/ISP-Dial/Security/SP/Voice)
bdennis_at_ine.com
Internetwork Expert, Inc.
http://www.INE.com
On 01/31/2012 11:07 AM, Scott Strobeck 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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