Content

| 5.7 |  | Troubleshooting OSPF |
| 5.7.4 |  | ABR not generating Type 4 summary LSA |

One of the functions of a Type 4 summary LSA is to announce the reachability of an ASBR to other areas. The Type 4 LSA is not required if the ASBR exists in the same area.

The ASBR does not generate the Type 4 summary LSA if it is not connected to area 0. To generate a summary LSA of Type 3 or Type 4, a router must have a connection to area 0. As a result, the external routes will not be installed in the network.

Figure shows a network in which R3 redistributes RIP routes into OSPF. Figure shows that R1 is not installing the external route 200.200.200.0/24 into the routing table.

Debugs and Verification
The output of show ip ospf database external command in Figure shows that the route exists in the external OSPF database of router R1. However, the output of show ip ospf database asbr-summary command in Figure shows that there is no Type 4 LSA for this route.

The next logical step is to go on the ABR and see if it is indeed an ABR. If it is an ABR it will generate a summary LSA of Type 3 or Type 4. If it is not an ABR, it will not generate that summary. The output of the show ip ospf command in Figure shows that router R2, which is between two areas and may look like an ABR, is not identifying itself as an ABR. If R2 were an ABR, the output would display that is an “Area Border Router.”

> Note: All areas in an OSPF autonomous system must be physically or virtually connected to the backbone area (area 0). For a router to be an ABR, one of its interfaces must be part of area 0.

Solution
In this example, router R2 does not generate the Type 4 summary LSA because it is not connected to area 0. To generate a summary LSA of Type 3 or Type 4, a router must have a connection into area 0.

To solve this problem, router R2 must be connected to area 0, either physically or virtually by creating a virtual link, as shown in Figure . By configuring a virtual link on R2, the router is now virtually connected to area 0; therefore, it now considers itself an ABR. After connecting R2 to area 0, the output of show ip ospf shows that it is now an ABR.

After the configuration change on R2, as shown in Figure , R1 is now generating a Type 4 summary LSA into area 3. Because the Type 4 LSA is now being received, R1 installs the external route 200.200.200.0/24 into its routing table.