Route summarization allows multiple routes with the same prefix to be summarized into a single summary route. Only the summary route is advertised to other areas. This reduces both the routing table size and routing information transmitted between areas, improving device performance.
OSPF supports two types of route summarization:
Inter-area route summarization
Inter-area route summarization is performed on ABRs and applies to routes from within an AS. An ABR generates Network-summary (Type 3) LSAs by network segment while advertising routing information to other areas. If network segments are contiguous, the ABR can summarize them into a single segment. This allows the ABR to send only the summary LSA for all the specified network segments.
External route summarization
External route summarization is performed on ASBRs and is specific to external routes that are imported into OSPF. ASBRs can summarize imported AS-external (Type 5) LSAs within a summary address range. After an NSSA is configured, an ASBR must summarize imported NSSA (Type 7) LSAs with the summary address range.
Devices that are both ABRs and ASBRs summarize the Type 5 LSAs translated from Type 7 LSAs.
OSPF supports route filtering using routing policies, including route-policy, access-list, and prefix-list. By default, OSPF does not filter routes.
OSPF route filtering can be used to:
Import routes.
OSPF can import routes learned by other routing protocols. You can configure routing policies to filter imported routes, allowing OSPF to import only routes that match specific conditions.
Advertise imported routes.
An OSPF device advertises imported routes to its neighbors.
You can configure filtering rules to filter the routes to be advertised, but the rules take effect only on ASBRs.
Learn routes.
Filtering rules can be configured to allow OSPF to filter received intra-area, inter-area, and AS external routes.
After receiving routes, an OSPF device adds only the routes that match filtering rules to the local routing table. However, the device can advertise all routes from the OSPF routing table.
Learn inter-area LSAs.
You can configure an ABR to filter incoming Summary LSAs. This configuration takes effect only on ABRs because only ABRs can advertise Summary LSAs.
Difference between route filtering for inter-area LSA learning and that for route learning: The former directly filters incoming LSAs but the latter filters the routes calculated using LSAs to determine the routes to be added to the local routing table. In route filtering for route learning, all incoming LSAs are learned.
Advertise inter-area LSAs.
You can configure an ABR to filter outgoing Summary LSAs. This configuration takes effect only on ABRs.