Decoding OSPF: Understanding Packet Flow in 10 Steps

Ns3Edu Team

April 12, 2024

Neighbor Discovery: OSPF routers discover neighbors by sending Hello packets out of all active interfaces.
Neighbor Formation: Routers exchange Hello packets to establish neighbor relationships. This includes negotiating parameters like router ID and area ID.
Database Synchronization: Once neighbors are discovered, routers exchange Database Description (DBD) packets to synchronize their link-state databases.
Link-State Advertisement (LSA) Exchange: Routers exchange Link-State Advertisements (LSAs) containing information about their directly connected links and networks.
LSA Flooding: LSAs received from neighbors are flooded throughout the OSPF domain to ensure all routers have a consistent view of the network.
Shortest Path First (SPF) Calculation: Each router runs the Dijkstra algorithm to calculate the shortest path tree based on the information in its link-state database.
Routing Table Update: After SPF calculation, routers populate their routing tables with the best paths to reach each destination network.
Sending Updates: If there are any changes in the network topology, routers send updated LSAs to their neighbors, triggering the process again.
Route Advertisement: Routers advertise their routes to neighboring routers through OSPF update packets.
Loop Prevention: OSPF employs various mechanisms like split horizon and route poisoning to prevent routing loops and ensure efficient packet delivery.