Routing Protocols allow routers to dynamically advertise and learn routes, determine which routes are available and which are the most efficient routes to a destination. Routing protocols provide the layer 3 network state update and populate routing tables on the layer 3 switch/router. However, some popular layer three protocols, such as Internet Protocol (IP), Novell Internetwork Packet eXchange (IPX), and AppleTalk are called routed protocols, which transport data across the network.

Packet routing in the Internet is divided into two general groups: interior and exterior routing. Interior routing happens inside or interior to an independent network system. In TCP/IP terminology, these independent network systems are called autonomous systems. Within an autonomous system (AS), routing information is exchanged using an interior routing protocol chosen by the autonomous system's administration. The exterior routing protocols, on the other hand are used between the autonomous systems.

There are two main types of algorithms for routing: Distance Vector Routing and Link State Routing. Basically, Distance Vector protocols determine best path on how far the destination is, while Link State protocols are capable of using more sophisticated methods taking into consideration link variables, such as bandwidth, delay, reliability and load.
Distance Vector protocols judge best path on how far it is. Distance can be hops or a combination of metrics calculated to represent a distance value. The IP Distance Vector routing protocols still in use today are: Routing Information Protocol (RIP v1 and v2) and Interior Gateway Routing Protocol (IGRP).

A Link-state routing is a concept used in routing of packet-switched networks in computer communications. Link-state routing works by having the routers tell every router on the network about its closest neighbors. The entire routing table is not distributed from any router, only the part of the table containing its neighbors.

Some of the link-state routing protocols are the OSPF, IS-IS and EIGRP. NetWare Link State Protocol (NLSP) is also a link-state routing protocol, supporting IPX. This type of routing protocol requires each router to maintain at least a partial map of the network. When a network link changes state (up to down, or vice versa), a notification, called a link state advertisement (LSA) is flooded throughout the network. All the routers note the change, and re-compute their routes accordingly.

Distance-vector routing protocols are simple and efficient in small networks, and require little, if any management. However, they do not scale well, and have poor convergence properties, which has led to the development of more complex but more scalable link-state routing protocols for use in large networks.

Link State Routing protocols provide greater flexibility and sophistication than the Distance Vector routing protocols. They reduce overall broadcast traffic and make better decisions about routing by taking characteristics such as bandwidth, delay, reliability, and load into consideration, instead of basing their decisions solely on distance or hop count.

Related Terms: OSPF, IS-IS, EIGRP, NLSP, Distance Vector Routing, Link State Routing, NHRP, RIP, RSVP, VRRP, EGP, IGRP, NARP, Exterior Routing, Interior Routing