OSPF Neighbor Adjacency Process

OSPF Neighbor Adjacency Process

• The OSPF neighbor adjacency process involves 8 states: Down, Attempt, Init, 2-Way, ExStart, Exchange, Loading, and Full states.

Down State

• The initial state when OSPF is first configured on a router.
• In this state, the router sends out periodic Hello packets to discover other OSPF routers.
• The router does not have an entry in its neighbor table at this point.

Attempt State

• A rare state that occurs on non-broadcast multi-access links.
• On such links, the neighbor's IP address must be manually configured.
• The Hello packets are sent unicast to the manually configured IP address.

Init State

• Triggered when a Hello packet is received from another router.
• The receiving router adds the sender's Router ID to its Hello packets.

2-Way State

• Triggered when a Hello packet is received and the router sees its own Router ID in the packet.
• Confirms two-way reachability between the routers.

ExStart State

• Routers exchange DBD (Database Descriptor) packets to elect a master and slave.
• The master-slave election determines the reliable exchange of LSDB (Link-State Database) summaries.

Exchange State

• The master and slave election is complete, and the routers exchange LSDB summaries.
• The master sends a DBD packet with its LSDB summary, and the slave responds with an empty DBD packet.

Loading State

• Both routers have exchanged LSDB summaries and know the LSAs (Link-State Advertisements) in each other's database.
• Routers request and receive specific LSAs using the LSR-LSU-LSAck process.

Full State

• The final state where the LSDbs are synchronized, and the routers have learned everything they need to know from each other.
• The "happy state" where OSPF neighbor adjacencies should reach.

Note: LSA stands for Link-State Advertisement, LSR stands for Link-State Request, LSU stands for Link-State Update, and LSAck stands for Link-State Acknowledgement.

OSPF Neighbor Adjacency Process

• The OSPF neighbor adjacency process consists of 8 states: Down, Attempt, Init, 2-Way, ExStart, Exchange, Loading, and Full states.

Down State

• Initial state when OSPF is first configured on a router.
• Router sends periodic Hello packets to discover other OSPF routers.
• No entry in the neighbor table at this point.

Attempt State

• Occurs on non-broadcast multi-access links.
• Neighbor's IP address must be manually configured on such links.
• Hello packets are sent unicast to the manually configured IP address.

Init State

• Triggered when a Hello packet is received from another router.
• Receiving router adds the sender's Router ID to its Hello packets.

2-Way State

• Triggered when a Hello packet is received and the router sees its own Router ID in the packet.
• Confirms two-way reachability between the routers.

ExStart State

• Routers exchange DBD (Database Descriptor) packets to elect a master and slave.
• Master-slave election determines reliable exchange of LSDB (Link-State Database) summaries.

Exchange State

• Master and slave election complete, routers exchange LSDB summaries.
• Master sends DBD packet with its LSDB summary, and slave responds with an empty DBD packet.

Loading State

• Routers have exchanged LSDB summaries and know the LSAs in each other's database.
• Routers request and receive specific LSAs using the LSR-LSU-LSAck process.

Full State

• Final state where the LSDBs are synchronized, and routers have learned everything they need to know from each other.
• "Happy state" where OSPF neighbor adjacencies should reach.

Additional Terms

• LSA: Link-State Advertisement
• LSR: Link-State Request
• LSU: Link-State Update
• LSAck: Link-State Acknowledgement