Unlike EIGRP, OSPF does not automatically summarize at Classful boundaries. In addition to this, OSPF summarization is different for internal and external routes, which often results in confusions and misconfigurations. In Cisco IOS software, internal route summarization is configured by using the area [area ID] range [<address> <mask> [advertise | not-advertise] [cost <cost>] router configuration command on the Area Border Router (ABR). Following the configuration of the summary on the ABR, the following occurs:
- An intra-area route for the summary pointing to Null0 is installed into the routing table
- The specific Type 3 entries in the LSDB are replaced by the single Type 3 LSA
A common source of confusion with OSPF summarization is that the area…range is used on a non-ABR router instead of the ABR. When this happens, the command will be accepted; however, the configured range will be flagged as passive and will not be advertised by the router. This can be validated using the show ip ospf command as illustrated below:
| R2#show ip ospf | begin Area Area BACKBONE(0) Number of interfaces in this area is 1 Area has no authentication SPF algorithm last executed 00:00:59.544 ago SPF algorithm executed 17 times Area ranges are Number of LSA 3. Checksum Sum 0x016581 Number of opaque link LSA 0. Checksum Sum 0x000000 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 0 Flood list length 0 Area 2 Number of interfaces in this area is 0 Area has no authentication SPF algorithm last executed 00:00:59.548 ago SPF algorithm executed 5 times Area ranges are 11.0.0.0/16 Passive Advertise Number of LSA 1. Checksum Sum 0x00032F Number of opaque link LSA 0. Checksum Sum 0x000000 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 0 Flood list length 0 |
External route summarization is configured on the OSPF ASBR using the summary-address [<address> <mask> | prefix] [not-advertise] [tag <tag>] [nssa-only] router configuration command. The [<address> <mask>keywords are used to specify the summary network address and its subnet mask. After this configuration has been implemented, the following is performed on the ASBR:
- An intra-area route for the summary pointing to Null0 is installed into the routing table
- The specific Type 5 entries in the LSDB are replaced by the single Type 5 LSA
A common misconfiguration is using the summary-address command on an ABR or other non-ASBR router. Another common mistake is attempting to use the area…range command when summarizing external routes. Neither of these solutions will work as the summary-addresscommand should only be implemented on an ASBR.
If you issue this command on an ASBR and no valid addresses belong to the range specified, the summary will be assigned a metric of infinity (16777215) by the router. For example, assume the following configuration was implemented on an ASBR:
| R1(config)#router ospf 1 R1(config-router)#summary-address 150.0.0.0 255.255.0.0 R1(config-router)#end |
Assuming that there are no known networks in the 150.0.0.0/16 range, then the summary route is assigned the infinity metric which can be viewed using the show ip ospf summary-address command. Following is the output of this command on the same router:
| R1#show ip ospf summary-address
OSPF Process 1, Summary-address 150.0.0.0/255.255.0.0 Metric 16777215, Type 0, Tag 0 |