Content

| 5.10 |  | Troubleshooting Redistribution |
| 5.10.3 |  | Redistribution problems with OSPF |

When a router in OSPF does redistribution, it becomes an ASBR. The routes that are redistributed into OSPF could be directly connected routes, static routes, or routes from another routing protocol or another OSPF process.

The two most common problems associated with OSPF and redistribution are:

1. OSPF is not installing external routes in the routing table.
2. ASBR is not advertising redistributed routes.

OSPF is not installing external routes in the routing table
When OSPF redistributes any routes whether connected, static, or from a different routing protocol, it generates a Type 5 LSA for those external routes. These Type 5 routes are flooded into every OSPF router, with the exception of those in stub and NSSA areas. Sometimes, the problem is that the external routes are in the OSPF database but are not being installed in the routing table.

The most common causes of this problem are:

1. The forwarding address is not known through the intra-area or interarea route.
2. The ABR is not generating Type 4 Summary LSAs.

Both of these problems are discussed in the Troubleshooting OSPF section.

ASBR is not advertising redistributed routes
Whenever a route is known to be connected or static, or when any other routing protocol is redistributed into OSPF, an external LSA is generated for that route. If an OSPF router is not advertising the external route even after the redistribution, this indicates a problem on a router that is doing the redistribution. Mostly, the problem stems from configuration mistakes.

The most common causes of this problem are:

1. The subnets keyword is missing from the ASBR configuration.
2. distribute-list out command is blocking the routes.

Distribute list issues are discussed in the section on Common IGP Routing Protocol Issues, Causes and Solutions. The following is an example of a problem caused by the missing subnets keyword on the ASBR configuration.

When any protocol is redistributed into OSPF, if the networks that are being redistributed are subnets, the subnets keyword must be used under the OSPF configuration. If the subnets keyword is not added, OSPF will ignore all the subnetted routes when generating the external LSA.

The situation could arise when connected or static routes are being redistributed into or out of OSPF. In that case, the same rule applies: The subnets keyword must be entered to redistribute subnetted routes.

Figure shows a network setup that is redistributing into OSPF.

Debugs and Verification
Figure shows the output of show ip ospf database external for 132.108.3.0. The output shows no LSA information, which means that R1 is not even originating the external LSA for 132.108.3.0. Figure shows the OSPF configuration of R1, and that the redistribute rip command under OSPF is missing the subnets keyword.

Solution
The solution to this problem is to add the subnets keyword to the subnets command under OSPF. After this option has been added, OSPF will redistribute all of the routes that are subnetted; for example 131.108.3.0 with the /24 mask. Figure shows that R1 is now generating the external LSA for 132.108.3.0/24 and 132.108.4.0/24.