Content

| 5.7 |  | Troubleshooting OSPF |
| 5.7.5 |  | Forwarding address is not known through the intra-area or interarea route |

When OSPF learns an external LSA, it makes sure that the forwarding address is known through an OSPF intra-area or interarea route before it installs the route into the routing table. In accordance with the RFC 2328 standard, if the forwarding address is not known through an intra-area or interarea route, OSPF will not install the route in the routing table.

Figure shows a network with the following specifications:

- R3 is an ASBR and is redistributing RIP routes into OSPF.
- R4 is running RIP with R3.
- R4 is learning 200.200.200.0/24 through RIP.
- R2 is running OSPF with R3.
- R2 is the ABR.

The network is experiencing a problem of external routes not getting installed in the routing table. Figure shows the output of show ip route for 200.200.200.0/24 on router R1. This network resides in a RIP domain. Because RIP is being redistributed into OSPF on router R3, all OSPF routers should see this external OSPF route. However, router R1 is not seeing it in its routing table.

Debugs and Verification
Figure shows the external LSA for 200.200.200.0/24 on R1. The output of the show ip ospf database external command shows that the external LSA exists in the OSPF database, but the route is still not being installed in the routing table. Note the forwarding address involved in this external LSA.

Figure shows that the route to the forwarding address of 131.108.0.4, is known through an OSPF external route. The ABR is summarizing 131.108.0.0/24 with the area range command, so the more specific intra-area routes are summarized into one route. This range summarizes all routes under the 131.108.0.0/26 range.

Figure shows that the ASBR is doing the redistribution from RIP into OSPF. It also shows that the connected networks in the range of 131.108.0.0/26 are being redistributed into OSPF because RIP owns those connected routes. This configuration redistributes 131.108.0.4/26, which is a connected interface on router R3. This subnet covers the forwarding address that appeared in Figure .

Router R1 is seeing the forwarding address learned through OSPF external because R3 has redistribute connected under router ospf. This leaks a more specific route for the connected interfaces of R3. This also includes the forwarding address and subnet, which is 131.108.0.4/26. Also, the intra-area route for this subnet is suppressed by R2 because R2 is summarizing 131.108.0.0/16. Because the more specific route is always preferred, R1 prefers the more specific external route of 131.108.0.4/26 over the less specific summarized route of 131.108.0.0/16.

Solution
The problem can be solved in one of two ways:

1. Do not summarize at the ABR.
2. Filter the connected subnet from being redistributed into OSPF at the ASBR.

To implement the first solution, remove the area range command on the ABR.

To implement the second solution, add a filter to control the redistributed routes on the ASBR. A route-map is added to the redistribute rip command, as shown in Figure to prevent the route 131.108.0.0/26 from getting redistributed into OSPF, permitting only the 200.200.200.0/24 route.

Figures and show the changes in the R1 routing table for the external 200.200.200.0/24 route and that the forwarding address is now known through OSPF interarea instead of OSPF external.