Configuring Route Redistribution

We learn in this post how to configure route redistribution between OSPF, EIGRP, BGP and static routes. The first section depicts OSPF route redistribution into EIGRP. The second section gives an example of redistributing static routes into EIGRP. And lastly we see an example of redistributing BGP into OSPF.

Injecting OSPF prefixes into EIGRP

– On the ASBR, and after redistribution, there’s a change in EIGRP topology table. But no changes happen in the routing table:

before redistribution:

after redistribution:

route redistribution command of OSPF into EIGRP

The change happens on the other routers, i.e. DSW1 and DSW2.

Redistribute ospf match internal

Before this command, here’s DSW1 RIB:

Then:

route redistribution command of OSPF with metric specification and matching internal routes

The 10.1.1.8 prefix is a subnet that’s internal to OSPF, i.e. it was not redistributed into OSPF before arriving to R4. The 0.0.0.0/0 is also an OSPF Internal route (the default route that’s injected by R3 into the totally stub NSSA) . So it is redistributed into EIGRP too.

“redistribute ospf ..”, without specifying which route type we want, is equivalent to “redistribute ospf match internal”.

What if we redistribute a connected interface on R2 into OSPF?

route redistribution of connected subnets into OSPF

R4 is not aware of it, since it is a totally stub NSSA router.

If we add the commands “redistribute ospf match external 1” and “redistribute ospf match external2”, IOS displays them in a single line in the config:

OSPF route redistribution with metric modification, matching internal and external prefixes

Let’s say we only want to see the default route on DSW1 (and DSW2) that has been redistributed into EIGRP and not let 10.1.1.8 be redistributed. With Route-map technology, it is possible. We create an ACL that matches the default route. We create the route-map that uses ACL for traffic matching. We call the route-map during redistribution of OSPF into EIGRP:

OSPF route redistribution into EIGRP with the use of a route map

and here’s the new RIB on the distribution switch:

Inject connected subnets into OSPF

In a past exercise, we created a loopback on R2 and redistributed it into OSPF. Let’s check that the 2.2.2.0 prefix has a cost of 20 and an O E2 type on another router:

metric-type

route redistribution of connected subnets and specifying metric type

What if we omit the “subnets” keyword?
R2 now advertises only classful networks that show in its RIB. Otherwise, they are not redistributed:

We’ll create a loopback on R4. We’ll redistribute it as a connected interface into OSPF. Normally, this will create a LSA7 in area 34 that will be converted by R3 in LSA5:

We see the LSIDs of 3 LSA7 in OSPF database of R4. They correspond to the connected subnets that have been redistributed.
R3 OSPF database(partial):

LSA7 shows too in its database. The new comer here is LSA5.
We can also see the 2.2.2.0 subnet, that corresponds to the loopback of R2 that has been redistributed into OSPF, at R2. This shows only on R3 database, because R4 gets only a default route to subnets that are outside area 34.
Let’s see OSPF database on R2 (partial):

Obviously, we see subnet 2.2.2.0 as LSA5 because it is redistributed here on R2. 4.4.4.0 shows too. It has been learned from R3 (an ABR) as LSA5. Everything works correctly.

Inject BGP prefixes into OSPF

Always following the same lab topology, we’ll redistribute BGP into OSPF domain on R1 because it is an ASBR:

BGP route redistribution into OSPF

On R2, we see no redistributed routes. That’s because R1 redistributes only classful networks. Let’s look at R1 RIB:

Since R1 has, in its RIB, subnets of 209.65.200.0 and not the classful network, then nothing is redistributed.
We add the “subnet” keyword during redistribution of BGP into OSPF:

BGP route redistribution of exact subnets into OSPF

And we can see the subnets being redistributed correctly, with a metric of 1 by default. Whenever you need a refresher on your CCNA skills and at the same time review at your own pace, then I would recommend you check my CCNA study material page.

To deepen your knowledge on route redistribution, check also this link.

Leave a Comment