Hello Thangarasu,
OSPF hasn't a routing table but a link state database.
The link state database has one instance for each OSPF area to which the router is connected to ( the router has at least one interface in the area).
Within an area, the OSPF link state database using multiple data structures describes completely the topology of routers and links in the area.
First of all, each OSPF router generates a router LSA describing its own links in the area.
Each router is identified by a unique router-id that is a 32 bit number like an IPv4 address.
In each LAN segment an OSPF DR is elected. The OSPF DR originates a Network LSA to describe what routers are connected to the LAN segment ( list of OSPF Router-ids).
The generated LSAs are propagated to all routers in the area. Each router can draw the current network diagram of the whole area and can execute the SPF alfgorithm to find the best route for each IPv4 prefix.
When multiple OSPF areas are used, area 0 must be the central area to which all other areas connect to.
Routers with some interfaces in area 0 and other interfaces in a different area or different areas are called ABR (Area border routers).
ABR routers have a very important role as they perform activities to allow inter area communication: an ABR converts Router LSAs and Network LSAs in LSA of type 3 that are sent in area 0. In the opposite direction they receive LSA type 3 summary routes from area 0 and depending on the type of area connected they regenerate these routes into the local non backbone area.
Routes external to OSPF domain can be injected into the OSPF database by an ASBR ( autonoumus system boundary router) as LSA type 5. LSA type 5 have OSPF domain scope and they are propagated to all areas except some special types of areas ( stub, totally stub, NSSA or totally NSSA).
ABR nodes propagate LSA type 4 to inform about the ASBR nodes that are in areas to which the ABR are connected to. (this allows router to check if the ASBR node is valid when analyzing an external route)
At the end routes in the OSPF link state database are proposed for installation in the IP routing table where they can be compared with routes coming from other routing sources.
If the OSPF route is the best it is installed in the IP routing table.
The IP routing table is maintained by a process that is different from the the OSPF process. The IP routing table maintainer decides what routes are placed in the IP routing table.
for additional info
see
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094aaa.shtml
http://docwiki.cisco.com/wiki/Open_Shortest_Path_First
Hope to help
Giuseppe
