Interior And Exterior Gateway Protocol


An interior gateway protocol is a type of protocol used for exchanging routing information between gateways within an autonomous system. This routing information can then be used to route network-layer protocols like IP.


Interior gateway protocols are used inside an organization’s network and are limited to the border router Exterior gateway protocols are used to connect the different Autonomous Systems (ASs). A simple definition that fits most of the time defines the border router as a router that has a foot in two worlds: one going to the Internet and another that is placed inside the organization (hence the name, border router). The routers in Figure 6.7 show four different ASs that the routers in their own clouds support. Within each cloud one router makes the connection back to the ISP and its AS. These are the border routers for each AS and they must run the exterior gateway protocol (Border Gateway Protocol, or BGP, in this case) as well as an interior gateway protocol. Other configurations may exist, such as multiple paths out to the Internet, but this example helps to clarify the concept.


What are the two types of interior gateway protocols?Interior gateway protocols can be divided into two categories: distance-vector routing protocols and link-state routing protocols.

1.Distance vetor routing protocol

 distance-vector routing protocol in data networks determines the best route for data packets based on distance. Distance-vector routing protocols measure the distance by the number of routers, a packet has to pass, one router counts as one hop. Some distance-vector protocols also take into account network latency and other factors that influence traffic on a given route. To determine the best route across a network, routers, on which a distance-vector protocol is implemented, exchange information with one another, usually routiing tables plus hop counts for destination networks and possibly other traffic information. Distance-vector routing protocols also require that a router informs its neighbours of network topology changes periodically.

Distance-vector routing protocols use the Bellman-Ford Algorithm to calculate the best route. Another way of calculating the best route across a network is based on link cost, and is implemented through link state routing protocols.

The term distance vector refers to the fact that the protocol manipulates vectors (arrays of distances to other nodes in the network. The distance vector algorithm was the original

Distance vector routing protocols include the following:

  • Routing Information Protocol (RIP) for IP
  • Xerox Networking System’s XNS RIP
  • Novell’s IPX RIP
  • Cisco’s Internet Gateway Routing Protocol (IGRP)
  • DEC’s DNA Phase IV
  • AppleTalk’s Routing Table Maintenance Protocol (RTMP)

Common Characteristics

A typical distance vector routing protocol uses a routing algorithm in which routers periodically send routing updates to all neighbors by broadcasting their entire route tables.3

The preceding statement contains a lot of information. Following sections consider it in more detail.

Periodic Updates

Periodic updates means that at the end of a certain time period, updates will be transmitted. This period typically ranges from 10 seconds for AppleTalk’s RTMP to 90 seconds for Cisco’s IGRP. At issue here is the fact that if updates are sent too frequently, congestion may occur; if updates are sent too infrequently, convergence time may be unacceptably high.


In the context of routers, neighbors always means routers sharing a common data link. A distance vector routing protocol sends its updates to neighboring routers4 and depends on them to pass the update information along to their neighbors. For this reason, distance vector routing is said to use hop-by-hop updates.

Broadcast Updates

When a router first becomes active on a network, how does it find other routers and how does it announce its own presence? Several methods are available. The simplest is to send the updates to the broadcast address (in the case of IP, Neighboring routers speaking the same routing protocol will hear the broadcasts and take appropriate action. Hosts and other devices uninterested in the routing updates will simply drop the packets.

Full Routing Table Updates

Most distance vector routing protocols take the very simple approach of telling their neighbors everything they know by broadcasting their entire route table, with some exceptions that are covered in following sections. Neighbors receiving these updates glean the information they need and discard everything else.

Link-state routing protocol

Link-state routing protocols are one of the two main classes of routing protocols used in packet switching networks for computer communications, the other being distance-vector routing protocols. Examples of link-state routing protocols include Open Shortest Path First and Intermediate System to Intermediate System. 

Link-State Routing Protocol Features

  • Building the Same LSDB on Every Router.
  • Router LSA: Includes a number to identify the router (router ID), the router’s interface IP addresses and masks, the state (up or down) of each interface, and the cost (metric) associated with the interface.


The Exterior Gateway Protocol was a routing protocol used to connect different autonomous systems on the Internet from the mid-1980s until the mid-1990s, when it was replaced by Border Gateway Protocol.

Exterior Gateway Protocol (EGP) is a PROTOCOL for exchanging routing information between two neighbor gateway hosts (each with its own router) in a network of autonomous systems. EGP is commonly used between hosts on the Internet to exchange routing table information. The routing table contains a list of known routers, the addresses they can reach, and a cost metric associated with the path to each router so that the best available route is chosen. Each router polls its neighbor at intervals between 120 to 480 seconds and the neighbor responds by sending its complete routing table. EGP-2 is the latest version of EGP.

A more recent exterior gateway protocol, the Border Gateway Protocol (BGP), provides additional capabilities.

What is the difference between an interior gateway protocol and an external gateway protocol?

Interior Gateway Protocol (IGP) is a Routing Protocol which is used to find network path information within an Autonomous System. Exterior Gateway Protocol (EGP) is a Routing Protocol which is used to find network path information between different Autonomous Systems.

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