So what makes the OSPF (Open Shortest Path First) different from most of the other interior gateway protocols you’ll encounter on your CCNA exam? For starters it is the only true link-state routing protocol. Link-state routing differs from distance-vector in many ways. The main difference is that routers running link-state protocol sends the state of their link to its neighbors until every router within the same link-state network has the same information. This requires a fairly complicated algorithm to accomplish this task. OSPF uses the SPF (Shortest Path First, hence the protocol name) created by the mastermind Dijkstra.
OSPF has many advanced features right out of the gate, such as using Variable Length Subnet Masks (VLSM), multicasts, and Classless Inter-domain Routing (CIDR). The multicast address that OSPF uses for routing updates is 188.8.131.52 and 184.108.40.206. The administrative distance for OSPF is 110 which are higher than EIGRP (90) but lower than RIP (120).
The OSPF protocol creates 3 separate tables before routing can begin within the OSPF network. The neighbor table, topology table, and the routing table are all used in conjunction in order to route packets efficiently. The neighbor table contains the information of the directly connected networks. The topology table shows the multiple paths to reach a particular network. The routing table is the best path to reach that particular network.
When the OSPF is running on a router, the router uses the HELLO message for the discovery of new neighbors and to also maintain its neighbor table. Every 10 seconds a HELLO message is sent between point to point connections. It should be noted that when OSPF is running on a NBMA (Non-Broadcast Multi-Access) network such as Frame Relay, HELLO packets are sent every 30 seconds.