What is EGP (Exterior Gateway Protocol)?

EGP (Exterior Gateway Protocol) uses external neighbors to broadcast accessibility information to other autonomous systems (AS). An external routing protocol is designed to be used between two networks under the control of two different organizations.

What is EGP (Exterior Gateway Protocol)?

EGP Routing

EGP is a protocol used to exchange routing information between external gateways (not belonging to the same Autonomous System AS). Gateways can only transfer accessibility information for AS networks. The gateway should collect this information through an Interior Gateway Protocol (IGP) used to exchange information between gateways of the same AS.

How Does It Work?

It relies on periodic polling using the Hello / I Hear You message exchanges to monitor neighbors’ accessibility and survey requests for an update. It restricts external gateways by allowing them to advertise only accessible target networks in the gateway’s autonomous system.

In this way, an external gateway using EGP transmits information to its neighbors but does not announce accessibility information (gateways neighbor if it exchanges routing information) outside the autonomy system.


The gateway does not report routes to other autonomous systems for which it is not part.

The network administrator will decide which gateways inform which gateways on other systems. In practice, everyone is informed about this availability, because there is no common method for determining routes. Therefore, central gateways that take more than one route for a particular gateway cannot use distance information to know which route is the best.

EGP is used to report routes to central gateways and they create a location availability model that results in a tree topology. Only central gateways know how to access any network on the Internet.

EGP1 and EGP2 extend the protocol to introduce the concept of routing cost, not just its scope. This is done by scaling the distance information over which the limits of an autonomous system are exceeded.

The routing domain is a group of redirectors using a common Interior Gateways Protocol (IGP). One way to reduce the volume of information transferred is that a routing domain uses a Gateways selected to transmit address information with Gateways selected from other domains. Selected Gateways are considered external Gateways, and the protocol used between external Gateways is EGP.

Main Features

  • It supports a NAP (“Neighbor Acquisition Protocol). Both gateways can be considered neighbors if both are connected by a transparent network. EGP does not specify how one gateway initially decides that it wants to be the neighbor of the other. Instead, an Acquisition It sends an “Acquisition confirmation” message in response to its request, which is required to obtain routing information from another gateway.
  • It supports an NR (“Neighbor Accessibility”) protocol. The gateway uses it to protect real-time information about its neighbors’ accessibility. To this end, it provides two types of messages: a Hello message and an I Hear You message (reply to Hello).
  • It supports update messages (or NR messages) that carry forwarding information. Apart from responding to the survey request, the gateway is not required to send NR messages to another gateway.

Message Types

It defines 10 types of messages to perform three basic functions:

  • Ask for a podium to be neighbor
  • Positive response to the “acquisition request”
  • The negative response to “request to acquire”
  • Neighborhood relationship termination request
  • Confirmation of stopping requests
  • Asking a neighbor for a response if she is alive
  • Reply hello message
  • Network routing table request
  • Network accessibility information
  • Replying to a wrong message


  • The biggest disadvantage of this protocol is that it creates a tree-like structure, so if there are problems on the internet, gateways only know that there are problems with external gateways.
  • It is among the router in different autonomous systems.
  • It is usually directly linked.
  • The next tab points to itself.

An additional problem with EGP relates to the amount of information changed; As the number of IP networks known by NSFNET increases, the size of NR messages increases, and the time required to process them becomes important.

For these reasons, BGP-3 replaced the NSFNET trunk EGP. However, BGP-3 does not require NSFNET or any other trunk to play a central role.

Instead, BGP-3 sees the Internet as an AS collection and does not take into account the internal topology of a system alone or the IGP or IGPs it uses.

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