Single-area OSPF is useful in smaller networks but in larger networks multiarea OSPF is a better choice. Multiarea OSPF solves the issues of large routing table, large link-state database, and frequent SPF algorithm calculations, as shown in Figures 1 and 2.
The main area is called the backbone area (area 0) and all other areas must connect to the backbone area. Routing still occurs between the areas while many of the routing operations, such as recalculating the database, are kept within an area.
There are four different types of OSPF routers: Internal router, Backbone router, Area Border Router (ABR), and Autonomous System Boundary Router (ASBR). A router can be classified as more than one router type.
Link State Advertisements (LSAs) are the building blocks of OSPF. This chapter concentrated on LSA type 1 to LSA type 5. Type 1 LSAs are referred to as the router link entries. Type 2 LSAs are referred to as the network link entries and are flooded by a DR. Type 3 LSAs are referred to as the summary link entries and are created and propagated by ABRs. A type 4 summary LSA is generated by an ABR only when an ASBR exists within an area. Type 5 external LSAs describe routes to networks outside the OSPF autonomous system. Type 5 LSAs are originated by the ASBR and are flooded to the entire autonomous system.
OSPF routes in an IPv4 routing table are identified using the following descriptors: O, O IA, O E1 or O E2. Each router uses the SPF algorithm against the LSDB to build the SPF tree. The SPF tree is used to determine the best paths.
There are no special commands required to implement a multiarea OSPF network. A router simply becomes an ABR when it has two network statements in different areas.
An example of multiarea OSPF configuration:
R1(config)# router ospf 10
R1(config-router)# router-id 126.96.36.199
R1(config-router)# network 10.1.1.1 0.0.0.0 area 1
R1(config-router)# network 10.1.2.1 0.0.0.0 area 1
R1(config-router)# network 192.168.10.1 0.0.0.0 area 0
OSPF does not perform auto summarization. In OSPF, summarization can only be configured on ABRs or ASBRs. Interarea route summarization must be manually configured and occurs on ABRs and applies to routes from within each area. To manually configure interarea route summarization on an ABR, use the area area-id range address mask router configuration mode command.
External route summarization is specific to external routes that are injected into OSPF via route redistribution. Generally, only ASBRs summarize external routes. External route summarization is configured on ASBRs using the summary-address address mask router configuration mode command.
Commands that are used to verify OSPF configuration consist of the following:
- show ip ospf neighbor
- show ip ospf
- show ip ospf interface
- show ip protocols
- show ip ospf interface brief
- show ip route ospf
- show ip ospf database