Problem: Router Is Not Summarizing Interarea Routes-Cause: area range Command Is Not Configured on ABR

Problem: Router Is Not Summarizing Interarea Routes ‚ Cause: area range Command Is Not Configured on ABR

You must ensure that the area range command is configured on the correct router. Area range summarization can be done only on the ABR. In summarization, instead of originating separate LSAs for each network, the ABR originates summary LSAs to cover those ranges of addresses.

Sometimes, the network mask is configured wrong and summarization doesn't work because of the misconfiguration. When configuring the area range command, make sure that the summarization mask is in the form of a prefix mask rather than a wildcard mask (that is, 255.255.255.0 instead of 0.0.0.255).

Figure 9-83 shows a network suffering from this problem. In this example, the area range com-mand is configured on R1. This command should be configured only on the router that belongs to the area for which routes are being summarized. In addition, the router must be an ABR.

Figure 9-83. OSPF Network in Which a Router Is Not Summarizing Interarea Routes

Figure 9-84 shows the flowchart to follow to solve this problem.

Figure 9-84. Problem-Resolution Flowchart

Debugs and Verification

Example 9-232 shows that R1 has the area range command for summarization of area 3 routes. The command highlighted in Example 9-232 defines the range to be summarized. This means that any address in the range 131.108.3.0 ‚ 255 will be summarized into a single route ‚ 131.108.3.0/24. Also, the mask should be in the format of 255.255.255.0 instead of 0.0.0.255. The format 0.0.0.255 is used for the access list range, but in OSPF, the format should be 255.255.255.0.

The syntax of this area range command is correct, but the problem is that R1 is not an ABR. R1 indeed is included in area 0, but it is not connected to area 3 and thus cannot summarize area 3 routes.

Example 9-232 R1 Summarizes Area 3 Routes

 R1#  router ospf 1   network 131.108.2.0 0.0.0.255 area 0   area 3 range 131.108.3.0 255.255.255.0  

There is an easy way to check whether a router is summarizing the routes properly. Example 9-233 shows the output of show ip ospf, which 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. Also note that the number of interfaces in this area is 0, meaning that this router is not connected to area 3. As a result, summarization will not work properly.

Example 9-233 Area 3 Interfaces Are Defined as Passive

 R1#  show ip ospf  Area 3  Number of interfaces in this area is 0  Area has no authentication         SPF algorithm executed 1 times         Area ranges are            131.108.3.0/24  Passive  

Because summarization is not happening, R1 is receiving four routes in its routing table instead of one summarized route. Example 9-234 shows that R1 is receiving four routes in the routing table known through OSPF interarea.

Example 9-234 R1's Routing Table Shows Four Routes Instead of One Summarized Route

 R1#  show ip route  .. . O IA    131.108.3.0/26 [110/64] via 131.108.2.2, 00:01:35, Serial0 O IA    131.108.3.64/26 [110/64] via 131.108.2.2, 00:01:35, Serial0 O IA    131.108.3.128/26 [110/64] via 131.108.2.2, 00:01:35, Serial0 O IA    131.108.3.192/26 [110/64] via 131.108.2.2, 00:01:35, Serial0 

Solution

To solve this problem, the command must be configured on R2, which is connected to area 3 and also is an ABR. Example 9-235 shows that the area range command is configured on the ABR, which is R2.

Example 9-235 Configuring the area range Command on R2 (ABR)

 R2#  router ospf 1   network 131.108.3.0 0.0.0.255 area 3   network 131.108.2.0 0.0.0.255 area 0   area 3 range 131.108.3.0 255.255.255.0  

Again, after configuring the range, check the range status to see whether it is active or passive. If it appears active, the router properly is summarizing the routes. Example 9-236 shows the output of show ip ospf, which indicates that area 3 has area ranges defined and active.

Example 9-236 Determining the Area Range Status in Area 3

 R2#  show ip ospf  Area 3  Number of interfaces in this area is 4  Area has no authentication         SPF algorithm executed 1 times         Area ranges are  131.108.3.0/24 Active (64)  

After verifying that R2 indeed is summarizing and that the range is active, check R1's routing table to see whether R1 still is seeing four routes. Example 9-237 shows that after summarization, R1 starts receiving one route instead of four routes.

Example 9-237 R1 Receives a Single Summarized Route

 R1#  show ip route  .. .  O IA    131.108.3.0/24 [110/64] via 131.108.2.2, 00:01:35, Serial0