IGP, EGP, and Autonomous System Explained

Interior Gateway Protocol (IGP) and Exterior Gateway Protocol (EGP) are two types of IP routing protocols. If a routing protocol is designed and intended for use in a single autonomous system, it is an IGP protocol. If used between different autonomous systems, it is an EGP protocol.

What is an autonomous system?

An Autonomous System is a group of networks that is governed and controlled by a single administrative entity. For example, a network created by a single company, organization, corporation, or ISP is a single AS.

What are AS numbers?

An AS number is a unique identity of the AS on the Internet. To connect your AS to the Internet, you must obtain an AS number. The Internet Assigned Numbers Authority (IANA) has the worldwide right to assign AS numbers. It delegates this right to the organizations that assign public IP addresses. For example, in Asia, the Asia Pacific Network Information Centre (APNIC) assigns both IP addresses and AS numbers.

What is an IGP protocol?

An AS can contain multiple networks. To connect these networks, you use a routing protocol. The routing protocol used to connect networks within the same autonomous system is known as an IGP (Interior Gateway Protocol). RIPv1, IGRP, OSPF, EIGRP, RIPv2, and IS-IS are examples of IGP protocols. Since all networks in an autonomous system belong to the same administrative entity, you can configure any IGP protocol to connect them.

What is an EGP protocol?

An EGP protocol provides connectivity between different autonomous systems. Since different autonomous systems belong to different administrative entities, you cannot use a routing protocol of your choice to connect them. You must use a uniform routing protocol. A uniform routing protocol that connects different autonomous systems is known as an exterior gateway routing protocol. There is only one EGP protocol, and that is BGP. It connects all public autonomous systems on the Internet.

The following image shows an example of IGP and EGP implementation.

Classification of routing protocols

The following image categorizes routing protocols by their type.

Differences between interior and exterior gateway protocols

The following table compares and lists the differences between the Interior and Exterior gateway routing protocols.

RIP

RIP is one of the earliest routing protocols introduced. It is a distance vector routing protocol. It uses the number of hops (routers) in the path as the metric. It sends out a copy of its routing table to neighbors every 30 seconds and triggers updates whenever the route's metric changes. It has two versions: RIPv1 and RIPv2. RIPv2 is the updated version of RIPv1.

OSPF

OSPF is an improvement to RIP. It is a link-state routing protocol. It uses the cost of the path as the metric. It uses hierarchical design, trigger updates, link-state advertisement, and many other features to provide fast convergence and reliability.

IS-IS

IS-IS is a link-state routing protocol. It is a simple version of OSPF. ISPs and large internetworks that need a stable and straightforward routing protocol deploy it. It uses a Hello protocol to establish adjacencies and LSPs to exchange link-state information.

EIGRP

Cisco developed IGRP to provide a better routing protocol than RIPv1. To support modern infrastructure, Cisco updated IGRP to EIGRP. EIGRP is a hybrid protocol. It uses the features of both distance vector and link-state technologies to provide fast convergence and stability. IGRP and EIGRP are proprietary protocols. To use these protocols, you have to use all Cisco routers in your network.

BGP

BGP is a path vector protocol. It works between different AS. It maintains path information and dynamically updates the information incrementally. To maintain path information, it uses a separate routing table. There are two versions of BGP: iBGP and eBGP. iBGP provides routing within the same AS. eBGP provides routing between different AS.

Conclusion

This tutorial compared IGP with EGP and described their difference. Learning these differences is essential for any entry-level networking certification program. It also helps you understand how networks connect over the Internet and Intranet.