In my last column, I offered an overview of the basics of using routing
redistribution. Routing redistribution involves taking routes from one
source (usually a routing protocol) and sending those routes to a different
source (usually another routing protocol).

Now that we’ve discussed exactly what routing redistribution
is, let’s look at a practical, hands-on example of using routing redistribution.
Last time, we discussed three situations in which one might use routing
redistribution. One of those scenarios involved the use of devices that don’t
support the routing protocol of your network.

If you’re using such devices, you’ll likely want to bring
routes from the devices into the routing protocol of your network. Let’s drill
down into this situation, and examine the proper configuration for each of the
devices.

Let’s say your organization’s network has a firewall that
only offers Routing Information Protocol (RIP)
routing, and you want that firewall to be the default gateway for the network.
However, the network is running Open Shortest Path First (OSPF) routing.

How do you get the RIP default route into your OSPF network?
The answer is redistribution. First, let’s look at a diagram of the network, as
shown in Figure A.

Figure A

The firewall is near the top of the diagram, and it connects
to Router 1. Router 1 then connects to Router 2. (In a real-world network, it
likely connects to other routers as well.)

Router 1 is our redistribution point, from which we’ll
redistribute the RIP routes we receive from the firewall into our OSPF network.
So, Router 1 is running both RIP and OSPF; it runs RIP on the interface
connecting to the firewall, and it runs OSPF on the interface connecting to
Router 2.

When we’re finished, we’ll see the default route on Router
2, originally sent by the firewall. And Router 2 will only have the default
route when Router 1 has the default route. Here’s something important to note: While
you could continually send the default route from Router 1 to Router 2 without
redistribution, you only want Router 2 to have the default route if it’s really
available.

Here’s the IP address configuration for Router 1:

interface Ethernet0
 ip address 1.1.1.125 255.255.255.0
!
interface Serial0
 ip address 2.2.2.1 255.255.255.0

Here’s the routing configuration for Router 1:

router ospf 1
 redistribute rip subnets
 network 2.2.2.0 0.0.0.255 area 0
 default-information originate
!
router rip
network 1.0.0.0

This shows us that the router has OSPF enabled on the Serial
interface and RIP enabled on the Ethernet interface. In addition, the following
commands are particularly important to note.

  • redistribute rip subnets: Under the
    OSPF routing process, this command sends any routes received from RIP to
    OSPF. The subnets keyword tells
    the router to redistribute all networks, including subnetted networks. By
    default, this command only redistributes classful networks.
  • default-information originate: Under
    the OSPF process, this command allows OSPF to advertise a default route.
    By default, this command only advertises the default route if the router
    really has a default route. However, you can use the always keyword at the end of the command to tell the router to
    advertise a default route via OSPF no matter what.

Now, let’s look at the IP address configuration for Router
2:

interface Ethernet0
 ip address 10.1.1.1 255.255.255.0
!
interface Serial0
 ip address 2.2.2.2 255.255.255.0
 clockrate 250000

Here’s the routing configuration for Router 2:

router ospf 1
network 10.1.1.0 0.0.0.255 area 0
 network 2.2.2.0 0.0.0.255 area 0

Note that Router 2 is only running OSPF.

Next, let’s use the show
ip route command on Router 1. Here’s a look at this command’s output:

R1# show ip route 
Codes: C - connected, S - static, R - RIP, M - mobile, B – BGP
 D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
 N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
 E1 - OSPF external type 1, E2 - OSPF external type 2
 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
 ia - IS-IS inter area, * - candidate default, U - per-user static route
 o - ODR, P - periodic downloaded static route

Gateway of last resort is 1.1.1.126 to network 0.0.0.0

 1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Ethernet0
 2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Serial0
 10.0.0.0/24 is subnetted, 1 subnets
O 10.1.1.0 [110/74] via 2.2.2.2, 00:13:19, Serial0
R* 0.0.0.0/0 [120/1] via 1.1.1.126, 00:00:03, Ethernet0

Notice that Router 1 is receiving the default route via RIP,
as indicated by the R next to the
0.0.0.0/0 default route at the bottom of the output. In addition, notice that
Router 1 is communicating with Router 2 via OSPF because it has the 10.1.1.0/24
route for the Ethernet LAN on Router 2.

Now, let’s use the show
ip route command on Router 2. Here’s a look at this command’s output:

R2# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B – BGP
 D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
 E1 - OSPF external type 1, E2 - OSPF external type 2, E – EGP
 i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
 U - per-user static route

Gateway of last resort is 2.2.2.1 to network 0.0.0.0

 1.0.0.0/8 is subnetted, 1 subnets
O E2 1.1.1.0 [110/20] via 2.2.2.1, 00:12:39, Serial0
 2.0.0.0/8 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Serial0
 10.0.0.0/8 is subnetted, 1 subnets
C 10.1.1.0 is directly connected, Ethernet0
O*E2 0.0.0.0/0 [110/1] via 2.2.2.1, 00:12:31, Serial0
R2#

Notice that Router 2 has the 0.0.0.0/0 default route
pointing to Router 1 (in this case, 2.2.2.1) and that the route has the O designation, which indicates the OSPF
route.

This is an external OSPF route, as indicated by E2. This shows that its source was not natively
OSPF, and it also indicates redistribution via some other protocol.

Router 2 has no RIP routes because it isn’t running RIP.
Through redistribution, however, it can receive the RIP route, originally
advertised by the firewall.

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David Davis has worked
in the IT industry for 12 years and holds several certifications, including
CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of
systems/network administrators for a privately owned retail company and
performs networking/systems consulting on a part-time basis.