6.17 Exchanging and Synchronizing LSDBs

Here is the process for forming neighbor adjacency and synchronizing link state databases:

The interface comes up on the local router.
Local router sends its hello packet to the multicast address 224.0.0.5, which includes the RID. These hello packets are sent out of every OSPF-enabled interface – State: Down > Init
Neighbors receive the initial hello and add the sending router to their list of neighbors – State: Down > Init
Neighbors respond via unicast with their own hello packets. Local router transitions to 2-Way (State: Init > 2-Way)
Local router replies with a unicast hello packet to the neighbor(s) unicast address to complete the introduction process. Remote routers transition to 2-Way.
Election to determine DR/BDR in broadcast networks.
Election to determine who sends first DBD packets between two routers. (DBD packets contain headers for all LSDB entries, not full information.) Router with the highest RID wins and sends first updates – State: 2-Way > Exstart

The DBD packets are reviewed by the recipient, in this case the local router, and then the local router passes on its own DBD summaries to neighbors. State: Exstart > Exchange
LSAck is sent between neighbors to acknowledge the receipt of the DBD transfers.
(LSR) Each router sends a request to neighbors for the complete entry for any missing routes. State: Exchange > Loading
(LSU) Each router sends link state update in response to any LSRs received from neighbor routers.
(LSAck) Each update must be acknowledged. State: Loading > Full
OSPF databases are now completely synced across the area and routers run their SPF algorithm.

DBD (Database Description packets) carry the following information: