Content

| 5.7 |  | Troubleshooting OSPF |
| 5.7.6 |  | Route summarization problems |

Route summarization allows a group of contiguous networks to be summarized as fewer addresses in order to help reduce the size of the routing table, thus increasing the performance of OSPF and other routing protocols.

OSPF can use two types of summarization:

1. Interarea route summarization that can be done on the ABR.
2. External route summarization that can be done on the ASBR.

Figure shows a network in which a router is not summarizing interarea routes.

Debugs and Verification
Router R1 has the area range command for summarization of area 3 routes. The syntax of the command is correct, but the problem is that R1 is not an ABR. Router R1 is included in area 0, but it is not connected to area 3, and thus cannot summarize area 3 routes.

A way to check if a router is summarizing the routes properly is to use the show ip ospf command. The output of show ip ospf shows that the area 3 range is passive. Passive means that no addresses within this area fall within this range. In fact, R1 does have the routes in this range; however, because R1 is not connected to area 3, the range appears as passive.

Because summarization is not happening, R1 is receiving four routes in the routing table instead of one summarized route.

Solution
The solution to this problem is to configure the area range command on the ABR, which is R2. After changing the configuration and removing the area range command on R1, verify that the summarization is indeed active by using show ip ospf on R2. The summarized route is now being installed in the routing table of R1.

> Note: There are similar issues with external summarization, which is done on the ASBR. External summarization uses the summary-address command only on the ASBR. The show ip ospf summary-address command can be used to help troubleshoot external summarization issues. A metric of infinity, shown in Figure , means that no valid addresses belong to this range of summarization and that there is a problem (16,777,215 equates to infinity because the external LSA metric is 24 bits long and 224 is equal to 16,777,215). Figure shows the same show ip ospf summary-address command, when summarization is configured correctly on the ASBR, with a metric less than infinity.