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Routing protocols discover routes on interconnected networks and build routing tables that provide routers with packet-forwarding information. Dynamic routing protocols (as opposed to manually configured static routing) automatically discover routes and create routing tables without operator intervention. Since network topologies are subject to change at any time (a link may fail), dynamic protocols are essential for routing around failed links in large internetworks.

Routing protocols may use distance-vector routing or link-state algorithms (also called shortest path first or SPF algorithms). Distance-vector routing can best be described as forwarding packets by getting directions along the way. Link-state routing is a better technique for larger networks. Routers use it to build a topological database that describes routes on the entire internetwork. This information is used to build routing tables with more accurate routing information. Link-state routing also responds faster to changes in the network. Link-state routing is now the preferred routing method for most organizations and Internet service providers.

The most common link-state routing protocol is OSPF (Open Shortest Path First). Other link-state protocols include IS-IS (Intermediate System to Intermediate System), an OSI protocol, and Novell's NLSP (NetWare Link Services Protocol). Distance-vector routing includes RIP (Routing Information Protocol), Cisco's IGRP (Internet Gateway Routing Protocol), and Apple's RTMP (Routing Table Maintenance Protocol).

OSPF is an internal routing protocol used within autonomous systems, such as service provider networks, universities, and private companies. Exterior routing protocols operate between autonomous systems. BGP (Border Gateway Protocol) is an exterior routing protocol.

The most important concept for link-state routing is that routers gather information about routes over the entire network. Link-state routers gather this information from neighbors and pass it on to other neighbors. Eventually, all the routers have information about all the links on the network. Then, each router runs the Dijkstra shortest path algorithm to calculate the best path to each network and create routing tables.

As mentioned, link-state routing responds faster to broken links or the addition of links. Routes can be based on the avoidance of congested areas, the speed of a line, the cost of using a line, or various priorities. OSPF (Open Shortest Path First) is the most common routing protocol to use the link-state algorithm. Refer to the OSPF topic for more details about the operation of link-state routing.

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