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 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 info for AS networks.

The gateway should collect this data through an Interior Gateway Protocol (IGP) used to exchange information between gateways of the same AS.

How Does EGP Protocol Work?

The reachability of these neighbors is monitored periodically. It also polls for update requests. It uses the “Hello” and “I Hear You” messages for this. We also restrict external gateways to advertise only reachable target networks in the autonomous system.

With this method, a gateway using the EGP protocol transmits information to its neighbors. However, it does not advertise its reachability info outside the AS. In short, these devices are considered neighbors if there is an exchange of reachability info.

Functions

The gateway does not advertise routes to autonomous systems that it is not part of. Also, the network administrator decides which gateways will notify devices in other systems. In practice, everyone gets this info about availability because there is no standard method for determining routes.

As a result, central gateways cannot use distance information to understand which route is best. We use EGP to advertise routes to central devices. This creates a model that results in a tree topology. Only central access points know how to access networks on the Internet.

EGP1 and EGP2 extend this to introduce the concept of routing cost. They also scale the routing cost with distance information. They do this across autonomous system boundaries.

A routing domain is a group of routers that use a common IGP. A routing domain communicates with selected gateways to reduce the volume of information it transfers. These selected gateways are also known as external devices. In other words, the standard used between external ones is the EGP protocol.

Main Features

• It supports a NAP (“Neighbor Acquisition Protocol”). We refer to two devices as neighbors if a transparent network connects them. EGP does not specify how a gateway initially decides that it wants to be the neighbor of the other. Instead, it sends an “Acquisition acknowledgment” message in response to an Acquisition Request. This step is required to obtain routing information from another gateway.
• It supports an NR (“Neighbor Reachability”) standard. The gateway uses it to maintain real-time info about the reachability of its neighbors. It provides two types of messages for this purpose: a Hello message and a Hear You message (a reply to Hello).
• It supports update messages (or NR messages) that carry routing information. The gateway is not required to send NR messages to another gateway except in response to a poll request.

Message Types

It defines ten types of messages to perform three essential functions:

• Ask for a podium to be a neighbor.
• Positive response to the “acquisition request.”
• The adverse reaction 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 info.
• Replying to a wrong message.

Disadvantages

• The most significant 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 devices.
• It is among the routers in different autonomous systems.
• It is usually directly linked.
• The next tab points to itself.

An additional problem with EGP is the amount of information exchanged. As the number of IP networks known to NSFNET increases, the size of the NR messages also increases. Therefore, the time required to process them becomes significant.

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

Instead, BGP-3 views the Internet as a collection of ASes. It does not take into account the internal topology of a single system or the IGP or IGPs it uses.