1-3z LSAck (link state acknowledgment) packet: Acknowledges received LSU packets. It contains theheaders of received LSAs (a packet can acknowledge multiple LSAs).LSA typesOSPF sends routing information in LSAs, which, as defined in RFC 2328, have the following types:z Router LSA: Type-1 LSA, originated by all routers, flooded throughout a single area only. This LSAdescribes the collected states of the router's interfaces to an area.z Network LSA: Type-2 LSA, originated for broadcast and NBMA networks by the designated router,flooded throughout a single area only. This LSA contains the list of routers connected to thenetwork.z Network Summary LSA: Type-3 LSA, originated by ABRs (Area Border Routers), and floodedthroughout the LSA's associated area. Each summary-LSA describes a route to a destinationoutside the area, yet still inside the AS (an inter-area route).z ASBR Summary LSA: Type-4 LSA, originated by ABRs and flooded throughout the LSA'sassociated area. Type 4 summary-LSAs describe routes to ASBR (Autonomous System BoundaryRouter).z AS External LSA: Type-5 LSA, originated by ASBRs, and flooded throughout the AS (except stuband NSSA areas). Each AS-external-LSA describes a route to another AS.z NSSA LSA: Type-7 LSA, as defined in RFC 1587, originated by ASBRs in NSSAs (Not-So-StubbyAreas) and flooded throughout a single NSSA. NSSA LSAs describe routes to other ASs.z Opaque LSA: A proposed type of LSA, the format of which consists of a standard LSA header andapplication specific information. Opaque LSAs are used by the OSPF protocol or by someapplication to distribute information into the OSPF routing domain. The opaque LSA includes threetypes, Type 9, Type 10 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 the local area, and theType 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. Neighbor relationships aremaintained by, and usually dynamically discovered by, OSPF's hello packets. When a router starts, itsends a hello packet via the OSPF interface, and the router that receives the hello packet checksparameters carried in the packet. If parameters of the two routers match, they become neighbors.Adjacency: A relationship formed between selected neighboring routers for the purpose of exchangingrouting information. Not every pair of neighboring routers become adjacent, which depends on networktypes. Only by synchronizing the LSDB via exchanging DD packets and LSAs can two routers becomeadjacent.OSPF Area PartitionArea partitionWhen a large number of OSPF routers are present on a network, LSDBs may become so large that agreat amount of storage space is occupied and CPU resources are exhausted by performing SPFcomputation.
