IGP and EGP / Link state and distance vector

IGP and EGP
10.2.7 This page will introduce two types of routing protocols.


An autonomous system is a network or set of networks under common administrative control, such as the cisco.com domain. An autonomous system consists of routers that present a consistent view of routing to the external world.

Two families of routing protocols are Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs).

IGPs route data within an autonomous system:

• RIP and RIPv2
• IGRP
• EIGRP
• OSPF
• Intermediate System-to-Intermediate System (IS-IS) protocol

EGPs route data between autonomous systems. An example of an EGP is BGP.

The next page will define link-state and distance vector protocols.

Link state and distance vector
10.2.8 Routing protocols can be classified as either IGPs or EGPs. Which type is used depends on 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. This page describes distance-vector and link-state routing and explains when each type of routing protocol is used.


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).

The Interactive Media Activity will identify the differences between link-state and distance vector routing protocols.

The next page will discuss routing protocols.