Open Shortest Path First Protocol

OSPF is very different from RIP. A router running RIP sends information about the entire network to its neighbors. A router running OSPF floods information about its neighbors to the entire network. Flooding means that the router sends the update out of every network port, and every router that receives the update also sends it out of every port except the one it receives it on. Flooding rapidly disseminates routing information to the entire network.

OSPF is called a link-state protocol because it creates a graph of the state of all of the links in the network. Every OSPF router creates its graph using the information about all of the routers and their neighbors that flooding distributes throughout the network. Each graph is unique because every router creates the graph with itself as the root of the tree. The graph is built using the Dijkstra Shortest Path First algorithm, hence the name of the protocol. The algorithm builds the graph in this manner:

1. The system starts by installing itself as the root of the graph with a cost of 0.

2. The system installs the neighbors of the system that was just added to the graph. The cost of reaching those neighbors is calculated as the cost of reaching the system just installed plus the cost that system advertises for reaching the neighbors.

3. The system selects the lowest-cost path for each destination. It repeats steps 2 and 3 for every system for which it has information.

Clearly, building a link-state graph for a large network every time a route changes creates a lot of overhead for the router. For this reason, OSPF divides the routing domain up into smaller, more manageable pieces. The entire routing domain is called an autonomous system, and the pieces are called areas. A special area, called the backbone area, is defined to interconnect all of the areas in the autonomous system. Routers within an area only have knowledge of their area and therefore only create a graph of the systems in that area.

OSPF is a much more complex system than RIP, but OSPF is better suited for large networks. However, OSPF is not always needed for an enterprise network, and may not be needed by a small network that uses Linux servers for routers. You may find that RIP is adequate for your needs.

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