Once neighbors have been identified, adjacencies must be established so that routing (LSA) information can be exchanged. There are two steps required to change a neighboring OSPF router into an adjacent OSPF router:
- Two-way communication (achieved via the Hello protocol)
- Database synchronization, which consists of three packet types being exchanged between routers:
- Database Description (DD) packets
- Link-State Request (LSR) packets
- Link-State Update (LSU) packets
Once database synchronization is complete, the two routers are considered adjacent.
It is important to remember that neighbours will not form adjacency if the following do not match
- Hello and dead Intervals
- Area ID
An ABR is a router that belongs to more than one OSPF area. Maintains information from all directly connected areas in its topology table and doesn’t share the topological details between areas.
It will forward only routing information from one area to another.ABR separates LSA flooding zone, primary point for summarization and maintains LSDB for each area its connected to.
ASBR is an OSPF router with at least one interface connected to external network or different AS.
An ASBR is responsible for injecting route information learned via the external network into OSPF and doesn’t do automatically, but done through route distribution.
Link State Advertisements
Router LSA (Type 1) – Contains a list of all links local to the router, and the status and “cost” of those links. Type 1 LSAs are generated by all routers in OSPF, and are flooded to all other routers within the local area.
Network LSA (Type 2) – Generated by all Designated Routers in OSPF, and contains a list of all routers attached to the Designated Router.
Network Summary LSA (Type 3) – Generated by all ABRs in OSPF, and contains a list of all destination networks within an area. Type 3 LSAs are sent between areas to allow inter-area communication to occur.
ASBR Summary LSA (Type 4) – Generated by ABRs in OSPF, and contains a route to any ASBRs in the OSPF system. Type 4 LSAs are sent from an ABR into its local area, so that Internal routers know how to exit the Autonomous System.
External LSA (Type 5) – Generated by ASBRs in OSPF, and contain routes to destination networks outside the local Autonomous System. Type 5 LSAs can also take the form of a default route to all networks outside the local AS. Type 5 LSAs are flooded to all areas in the OSPF system