Open Shortest Path First (OSPF) Protocol States

Open Shortest Path First (OSPF) is a link-state routing protocol used to find the best path between routers within an Autonomous System (AS). OSPF routers establish neighbor relationships to exchange routing information, and these relationships play a crucial role in the formation of OSPF adjacencies. Understanding the transitions between OSPF protocol states is essential for optimizing network performance and troubleshooting.

• OSPF uses Hello packets for neighbor discovery and to establish communication between routers.
• Neighbor states are key in forming OSPF adjacencies, enabling efficient routing.
• The main focus of this article is to understand OSPF state transitions and their significance in the network.
• OSPF requires specific parameters like matching Area IDs, authentication, MTU, and timers for adjacencies to form.
• Distinguishing between OSPF neighbors and adjacent routers is critical for proper network operation.
• Proper management of OSPF states ensures stable and optimized packet routing.

Open Shortest Path First (OSPF) Terms

• Router ID: The Router ID is the highest active IP address on the router. The process first considers the highest loopback address. If no loopback address is configured, the highest active IP address on the router's interfaces is used as the Router ID.

• Router Priority: The router priority is an 8-bit value assigned to a router running OSPF, which is used during the election of the Designated Router (DR) and Backup Designated Router (BDR) in a broadcast network.

• Designated Router (DR): The Designated Router (DR) is elected to minimize the number of OSPF adjacencies in a network. It is responsible for distributing Link State Advertisements (LSAs) to all other routers. In a broadcast network, the DR handles LSR (Link State Request) messages and responds with updates.

• Backup Designated Router (BDR): The Backup Designated Router (BDR) serves as a backup to the DR in a broadcast network. If the DR fails, the BDR assumes the role of DR and takes over its responsibilities.

• DR and BDR Election: The election of the DR and BDR occurs in broadcast or multi-access networks. The following criteria are used for the election:
  1. The router with the highest priority is elected as the DR.
  2. If there is a tie in the router priority, the router with the highest Router ID is chosen. The Router ID is first determined by the highest loopback address, and if no loopback is configured, the highest active IP address on the router’s interfaces is considered.

Open Shortest Path First (OSPF) States 

The device operating OSPF goes through certain states. These states are: 

Down State

• No Hello packets have been received on the interface.
• This is the initial state before communication begins between routers. The OSPF adjacency process has not started yet.

Note: The Down state does not imply that the interface is physically down; it simply means that the OSPF router has not started the process of forming adjacencies.

Init State

• The router receives a Hello packet from another router but is not yet listed in the neighbor’s Hello packet.
• The router has begun communication, but no bidirectional communication has been established yet.

Two-Way State

• Bi-directional communication is confirmed when each router sees itself in the other's Hello packet.
• In broadcast or multi-access networks, this is the point at which the Designated Router (DR) and Backup Designated Router (BDR) election takes place.

ExStart State

• The Master/Slave relationship is established for Database Description (DBD) exchange.
• Routers exchange initial DBD packets, which contain the sequence numbers, indicating the beginning of the exchange phase. The router with the higher Router ID becomes the master, while the other becomes the slave.

Exchange State

• In this state, routers exchange full Database Description (DBD) packets, which contain the headers of Link State Advertisements (LSAs).
• Routers create Link-State Request (LSR) lists to request missing LSAs that are not present in their own Link-State Database (LSDB).

Loading State

• Routers send Link-State Request (LSR) packets to request missing LSAs.
• The routers respond with Link-State Update (LSU) packets, which contain the requested LSAs.

Note: If a router receives a DBD from another router and finds that the received DBD is more up-to-date than its own, it will send an LSR to request missing links. The other router will reply with an LSU containing the missing updates, and a Link State Acknowledgment (LSA) is sent in return.

Full State

• The adjacency between routers is fully established.
• The Link-State Databases (LSDB) are synchronized, meaning both routers have identical databases.
• Routers can now compute the shortest paths using the Shortest Path First (SPF) algorithm, marking the point where routing decisions can be made.

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Article Tags :

• Misc
• Computer Networks
• GATE CS
• Computer Networks-Network Layer

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