OSPF: LSA Exchange

Once all DBD packets have exchanged all known LSA headers, the router needs to request the full copy of the LSA for each route its missing or needs to be updated in its own LSDB.

To know if an LSA has a more updated copy it examines the sequence number of the LSA header if the sequence number is higher in the received DBD thats what indicates that it is a newer route.

Routers use LSR packets to request one or more LSAs from the neighbor.  The neighboring router supplies LSUs in response to the LSRs containing all the detailed full LSA information the requesting router needs to update its LSDB.

LSR/LSA uses a reliable protocol that has two ways of acknowledging packets.  LSUs can be acknowledged by sending the exact same LSU back to the sender or a router can send back an LSAck back to the sender to acknowledge the packet, this LSAck contains a list of acknowledged LSA headers.

Once all LSAs have been fully exchanged between the neighbors, the router updates the neighbor state to Full and they can run the Djikstra SPF algorithm to determine best path.


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