OSPF Overview

OSPF is an interior gateway protocol (IGP) that routes packets within a single autonomous system (AS). In an OSPF network, the network topology is distributed among the nodes of the AS and is regularly updated through the exchange of link-state advertisements (LSAs). As a result, OSPF is known as a link-state protocol. Because topology changes are flooded throughout the network, every node maintains the same copy of the network map in its local topological database. Packets are then routed based on the shared topology using the shortest path first (SPF) algorithm.

Note: In this topic, the term OSPF refers to OSPF version 2 and OSPF version 3.

OSPF creates a topology map by flooding LSAs across OSPF-enabled links. LSAs announce the presence of OSPF-enabled interfaces to adjacent OSPF interfaces. The exchange of LSAs establishes bidirectional connectivity between all adjacent OSPF interfaces (neighbors) using a three-way handshake, as shown in Figure 14.

Figure 14: OSPF Three-Way Handshake

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In Figure 14, Router A sends hello packets out all its OSPF-enabled interfaces when it comes online. Router B receives the packet, which establishes that Router B can receive traffic from Router A. Router B generates a response to Router A to acknowledge receipt of the hello packet. When Router A receives the response, it establishes that Router B can receive traffic from Router A. Router A then generates a final response packet to inform Router B that Router A can receive traffic from Router B. This three-way handshake ensures bidirectional connectivity.

As new neighbors are added to the network or existing neighbors lose connectivity, the adjacencies in the topology map are modified accordingly through the exchange (or absence) of LSAs. These LSAs advertise only the incremental changes in the network, which helps minimize the amount of OSPF traffic on the network. The adjacencies are shared and used to create the network topology in the topological database.

OSPF Overview

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