Routing protocols can be classified as either IGPs or EGPs, which describes whether a group of routers is under a single administration or not. IGPs can be further categorized as either distance-vector or link-state protocols.

The distance-vector routing approach determines the distance and direction, vector, to any link in the internetwork. The distance may be the hop count to the link. Routers using distance-vector algorithms send all or part of their routing table entries to adjacent routers on a periodic basis. This happens even if there are no changes in the network. By receiving a routing update, a router can verify all the known routes and make changes to its routing table. This process is also known as “routing by rumor”. The understanding that a router has of the network is based upon the perspective of the adjacent router of the network topology.

Examples of distance-vector protocols include the following:

• Routing Information Protocol (RIP) – The most common IGP in the Internet, RIP uses hop count as its only routing metric.
• Interior Gateway Routing Protocol (IGRP) – This IGP was developed by Cisco to address issues associated with routing in large, heterogeneous networks.
• Enhanced IGRP (EIGRP) – This Cisco-proprietary IGP includes many of the features of a link-state routing protocol. Because of this, it has been called a balanced-hybrid protocol, but it is really an advanced distance-vector routing protocol.

Link-state routing protocols were designed to overcome limitations of distance vector routing protocols. Link-state routing protocols respond quickly to network changes sending trigger updates only when a network change has occurred. Link-state routing protocols send periodic updates, known as link-state refreshes, at longer time intervals, such as every 30 minutes.

When a route or link changes, the device that detected the change creates a link-state advertisement (LSA) concerning that link. The LSA is then transmitted to all neighboring devices. Each routing device takes a copy of the LSA, updates its link-state database, and forwards the LSA to all neighboring devices. This flooding of LSAs is required to ensure that all routing devices create databases that accurately reflect the network topology before updating their routing tables.

Link-state algorithms typically use their databases to create routing table entries that prefer the shortest path. Examples of link-state protocols include Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS).

Interactive Media Activity Checkbox: Link-State and Distance Vector Routing Protocols After completing this activity, the student will be able to identify the difference between link-state and distance vector routing protocols.

   

Web Links Distance Vector versus Link State http://www.inetdaemon.com/tutorials/ internet/routing/ dv_vs_ls.html