Adjacency

OSPF support two types of adjacency

        * Neighborship:
                the routers who stuck in 2-way state
                They do not have power to share updates to each other
                DRother routers are considered in Neighborship state
                DRother to DRother
                Whenever any DRother gets a update they send the update to DR on multicast address 224.0.0.6
        * Fully adjacent:
                The routers who pass all the states and goes in Full state
                Fully adjacent routers can share updates to each other
                        DR to BDR
                        DR to DRother
                        BDR to DRother

Router-ID

can be any IP (existing or non existing)
        1-manually defined //router-id 1.1.1.1
        2-highest loopback
        To present itself in an area and to tell the identity of the database, the router-id is forwarded in the hello packet and should be unique because two routers cannot have the same identity.
        Whenever we enable OSPF on a router, first task is always to select a router ID, if router do not find any active ipv4 address in global routing table it throw a error not to start OSPF process

We can manipulate the router-id by using a command in router configuration mode, if the router is having pre-existing neighbor then u have to clear OSPF process first, then the new router-d will will be in effect.