Introduction to OSPF 919LSA typesOSPF sends routing information in LSAs, which, as defined in RFC 2328, have thefollowing types:■ Router LSA: Type-1 LSA, originated by all routers, flooded throughout a singlearea only. This LSA describes the collected states of the router’s interfaces to anarea.■ Network LSA: Type-2 LSA, originated for broadcast and NBMA networks by theDesignated Router, flooded throughout a single area only. This LSA containsthe list of routers connected to the network.■ Network Summary LSA: Type-3 LSA, originated by ABRs (Area Border Routers),and flooded throughout the LSA’s associated area. Each summary-LSAdescribes a route to a destination outside the area, yet still inside the AS (aninter-area route).■ ASBR Summary LSA: Type-4 LSA, originated by ABRs and flooded throughoutthe LSA’s associated area. Type 4 summary-LSAs describe routes to ASBR(Autonomous System Boundary Router).■ AS External LSA: Type-5 LSA, originated by ASBRs, and flooded throughout theAS (except Stub and NSSA areas). Each AS-external-LSA describes a route toanother Autonomous System.■ NSSA LSA: Type-7 LSA, as defined in RFC 1587, originated by ASBRs in NSSAs(Not-So-Stubby Areas) and flooded throughout a single NSSA. NSSA LSAsdescribe routes to other ASs.■ Opaque LSA: A proposed type of LSA, the format of which consists of astandard LSA header and application specific information. Opaque LSAs areused by the OSPF protocol or by some application to distribute information intothe OSPF routing domain. The opaque LSA includes three types, Type 9, Type10 and Type 11, which are used to flood into different areas. The Type 9opaque LSA is flooded into the local subnet, the Type 10 is flooded into thelocal area, and the Type 11 is flooded throughout the whole AS.Neighbor and AdjacencyIn OSPF, the “Neighbor” and “Adjacency” are two different concepts.Neighbor: Two routers that have interfaces to a common network. Neighborrelationships are maintained by, and usually dynamically discovered by, OSPF’shello packets. When a router starts, it sends a hello packet via the OSPF interface,and the router that receives the hello packet checks parameters carried in thepacket. If parameters of the two routers match, they become neighbors.Adjacency: A relationship formed between selected neighboring routers for thepurpose of exchanging routing information. Not every pair of neighboring routersbecome adjacent, which depends on network types. Only by synchronizing theLSDB via exchanging DD packets and LSAs can two routers become adjacent.OSPF Area Partition andRoute SummarizationArea partitionWhen a large number of OSPF routers are present on a network, LSDBs maybecome so large that a great amount of storage space is occupied and CPUresources are exhausted performing SPF computation.
