WeOSOSPF Route Redistribution HowTo
About
This document aims to provide example OSPF use-cases where route redistribution is applied in order to distribute routes from other sources into the OSPF domain.
For an overview of all available configuration settings for OSPF refer to the configuration guide:
Introduction
Currently support exist to redistribute other RIP networks, connected routes and static routes into the OSPF domain. In addition routers are also able to inject a default route (0.0.0.0/0) into the the OSPF domain.
Any router who performs route redistribution into the OSPF domain is known as a Autonomous System Boundary Router (ASBR).
Note
Remember, if stub areas are used and it is necessary to redistribute routing information directly into that area, the type must be changed to NSSA. This is because a stub area cannot harbor any ASBRs.
The following example use-cases are presented in this document:
Case 1: Redistribute RIP
In Figure 1, an example setup consisting of one OSPF network and one RIP network
is displayed. In order to redistribute the RIP network into the OSPF network,
and vice versa, it needs to be configured on the ASBR.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. OSPF Network . . RIP Network .
. . . .
. .-------. . . .-------. .
. | | . . | | .
. | R1 | . . | R4 | .
. | | . . | | .
. '---+---' . . '---+---' .
. .2| . . |.2 .
. | 192.168.2.0/24 . . 172.16.2.0/24 | .
. .1| . . |.1 .
. .---+---. .-------. .---+---. .
. | |.1 .2| |.2 .1| | .
. | R2 +------------------+ ASBR +------------------+ R5 | .
. | | 192.168.1.0/24 | | 172.16.1.0/24 | | .
. '---+---' '-------' '---+---' .
. .1| . . |.1 .
. | 192.168.3.0/24 . . 172.16.3.0/24 | .
. .2| . . |.2 .
. .---+---. . . .---+---. .
. | | . . | | .
. | R3 | . . | R6 | .
. | | . . | | .
. '-------' . . '-------' .
. . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 1: Example of an OSPF network and a RIP network distributing its routes into each others domains.
When redistributed any destination will be reachable as if they were part of the same type of domain.
Configure
For this example we assume that both the OSPF network and the RIP network has been configured with the relevant internal networks. Refer to this example for basic OSPF network configuration, and to this for RIP network configuration.
Redistribution will be configured for both the OSPF and RIP networks, to redistribute each other.
ASBR
First we configure OSPF to redistribute the RIP network into its own domain:
ASBR:/#> configure ASBR:/config/#> router ASBR:/config/router/#> ospf ASBR:/config/router/ospf/#> redistribute rip ASBR:/config/router/ospf/#> leave ASBR:/#>
Then we can also configure the RIP network to redistribute the OSPF network into its own domain:
ASBR:/#> configure ASBR:/config/#> router ASBR:/config/router/#> rip ASBR:/config/router/rip/#> redistribute ospf ASBR:/config/router/rip/#> leave ASBR:/#>
Note
It may be necessary to also redistribute connected
routes to be able to reach the
networks 192.168.1.0/24 and 172.16.1.0/24 across the domains, because they
are directly connected to the ASBR. Directly connected routes have
administrative distance of 0 and will be selected above all other routes by
default, so the RIP or OSPF route for those networks will not be active.
Routes
When redistributed we should be able to see routes to all parts of the OSPF
domain, along with routes towards the networks present in the RIP domain. As an
example, the routes on R1, in the OSPF domain, looks like this:
R1:/#> show ip route S - Static | C - Connected | K - Kernel route | > - Selected route O - OSPF | R - RIP | [Distance/Metric] | * - FIB route O>* 172.16.1.0/24 [110/20] via 192.168.2.1, vlan1, 00:11:06 O>* 172.16.2.0/24 [110/20] via 192.168.2.1, vlan1, 00:10:58 O>* 172.16.3.0/24 [110/20] via 192.168.2.1, vlan1, 00:10:58 O>* 192.168.1.0/24 [110/20] via 192.168.2.1, vlan1, 00:31:25 O 192.168.2.0/24 [110/10] is directly connected, vlan1, 00:36:25 C>* 192.168.2.0/24 is directly connected, vlan1 O>* 192.168.3.0/24 [110/20] via 192.168.2.1, vlan1, 00:31:25
In the same manner, the routes on R4, in the RIP domain, looks like this:
R4:/#> show ip route S - Static | C - Connected | K - Kernel route | > - Selected route O - OSPF | R - RIP | [Distance/Metric] | * - FIB route R>* 172.16.1.0/24 [120/2] via 172.16.2.1, vlan1, 00:28:58 C>* 172.16.2.0/24 is directly connected, vlan1 R>* 172.16.3.0/24 [120/2] via 172.16.2.1, vlan1, 00:28:58 R>* 192.168.1.0/24 [120/3] via 172.16.2.1, vlan1, 00:12:06 R>* 192.168.2.0/24 [120/3] via 172.16.2.1, vlan1, 00:23:26 R>* 192.168.3.0/24 [120/3] via 172.16.2.1, vlan1, 00:23:26
Case 2: Redistribute Connected Routes
The case presented in Figure 2, shows a very simple example where a connected route is redistributed into an OSPF domain.
. . . . . . . . . . . . . . . . . .
. OSPF Network .
. .
. .-------. .-------. .-------.
. | |.2 .1| |.1 .2| |
. | R1 +------------------+ ASBR +------------------+ R2 |
. | | 192.168.1.0/24 | | 172.16.1.0/24 | |
. '-------' '-------' '-------'
. .
. .
. . . . . . . . . . . . . . . . . .
Figure 2: Example of an OSPF network redistributing a connected route into the OSPF domain.
Note
Remember that the router R2 also needs to have a route towards the ASBR if
it is to be reachable from inside the OSPF network. It could, for instance, be
configured with a static route or a default route towards the ASBR.
Configure
As in the previous case, we assume that the OSPF network itself is already configured as intended. Configuring a router to redistribute a connected route will also turn it into a ASBR.
ASBR
The router is configured to redistribute connected routes in the following manner:
ASBR:/#> configure ASBR:/config/#> router ASBR:/config/router/#> rip ASBR:/config/router/rip/#> redistribute connected ASBR:/config/router/rip/#> leave ASBR:/#>
Now routes to all connected links will be redistributed into the OSPF domain. In this example case we only have one connected link, but keep in mind that all connected links will be redistributed.
Routes
If the active routes are observed on R1, inside the OSPF domain, a route
towards the connected network can be observed:
R1:/#> show ip route S - Static | C - Connected | K - Kernel route | > - Selected route O - OSPF | R - RIP | [Distance/Metric] | * - FIB route O>* 172.16.1.0/24 [110/20] via 192.168.1.1, vlan1, 00:23:47 O 192.168.1.0/24 [110/10] is directly connected, vlan1, 00:25:23 C>* 192.168.1.0/24 is directly connected, vlan1
Case 3: Redistribute Static Routes
In Figure 3, an example case is presented where an OSPF network is connected to some external network. We want some of these networks to be accessible from within the OSPF domain. The ASBR have static routes configured to reach these networks that we want to redistribute into the OSPF domain.
Networks:
172.16.10.0/24
. . . . . . . . . . . . . . . . . . 172.16.20.0/24
. OSPF Network . 172.16.30.0/24
. .
. .-------. .-------. .-------. .--.-.
. | |.2 .1| |.1 .2| | ( ( )
. | R1 +------------------+ ASBR +---------------+ R2 +---(_, \ ) ,_)
. | | 192.168.1.0/24 | | 172.16.1.0/24 | | '-'--`--'
. '-------' '-------' '-------' External Network
. .
. .
. . . . . . . . . . . . . . . . . .
Figure 3: Example of an OSPF network redistributing static routes into the OSPF domain.
Configure
As before, we assume that the OSPF network itself is already configured as it is supposed to be. We will configure the static routes and their redistribution on the ASBR.
ASBR
First we add the static routes towards the intended networks, located in the
connected external network, through 172.16.1.2.
ASBR:/#> configure ASBR:/config/#> ip ASBR:/config/ip/#> route 172.16.10.0/24 172.16.1.2 ASBR:/config/ip/#> route 172.16.20.0/24 172.16.1.2 ASBR:/config/ip/#> route 172.16.30.0/24 172.16.1.2 ASBR:/config/ip/#> leave ASBR:/#>
When the static routes have been set up, OSPF can be configured to redistribute those routes in the following way:
ASBR:/#> configure ASBR:/config/#> router ASBR:/config/router/#> rip ASBR:/config/router/rip/#> redistribute static ASBR:/config/router/rip/#> leave ASBR:/#>
Routes
Looking at the routing table for R1 we can see that the static routes have
been distributed through the OSPF domain:
R1:/#> show ip route S - Static | C - Connected | K - Kernel route | > - Selected route O - OSPF | R - RIP | [Distance/Metric] | * - FIB route O>* 172.16.10.0/24 [110/20] via 192.168.1.1, vlan1, 00:00:01 O>* 172.16.20.0/24 [110/20] via 192.168.1.1, vlan1, 00:00:01 O>* 172.16.30.0/24 [110/20] via 192.168.1.1, vlan1, 00:00:01 O 192.168.1.0/24 [110/10] is directly connected, vlan1, 00:07:27 C>* 192.168.1.0/24 is directly connected, vlan1
Case 4: Redistribute Default Route
Distributing a default route cannot be done by redistributing a static route, it utilizes its own command.
. . . . . . . . . . . . . . . . . .
. OSPF Network .
. .
. .-------. .-------. .-------.
. | |.2 .1| |.1 .2| |
. | R1 +------------------+ ASBR +------------------+ GW |
. | | 192.168.1.0/24 | | 172.16.1.0/24 | |
. '-------' '-------' '-------'
. .
. .
. . . . . . . . . . . . . . . . . .
Figure 4: Example of an OSPF network distributing a default route into the OSPF domain.
The ASBR have a default route towards the GW router. The ASBR in turn
injects a default route into the OSPF network that points to itself.
Configure
Same as before, we assume that the OSPF network itself is already configured as
it is supposed to be. We configure the ASBR to advertise a default route into
the OSPF domain.
ASBR
Configure OSPF to advertise a default route into the OSPF domain:
ASBR:/#> configure ASBR:/config/#> router ASBR:/config/router/#> ospf ASBR:/config/router/ospf/#> distribute-default ASBR:/config/router/ospf/#> leave ASBR:/#>
Note
A default route will not be advertised by OSPF if it is not already present in
the routing table. If the always keyword is added, to the
distribute-default command, a default route will be advertised even if it is
not present in the routing table.
Routes
When the a default route is being advertised the routing table on R1 will look
like this:
R1:/#> show ip route S - Static | C - Connected | K - Kernel route | > - Selected route O - OSPF | R - RIP | [Distance/Metric] | * - FIB route O>* 0.0.0.0/0 [110/10] via 192.168.1.1, vlan1, 00:20:26 O 192.168.1.0/24 [110/10] is directly connected, vlan1, 00:20:37 C>* 192.168.1.0/24 is directly connected, vlan1