This tutorial explains interior gateway protocol (IGP), exterior gateway protocol (EGP), and autonomous system (AS) and lists the differences between IGP and EGP.
There are two categories of IP routing protocols: interior gateway protocol (IGP) and exterior gateway protocol (EGP). If a routing protocol is designed and intended for use in a single autonomous system, it falls under IGP. If a routing protocol is designed and intended for use between different autonomous systems, it falls under EGP.
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. If you want to connect your AS to the Internet, you must obtain an AS number. Internet Assigned Numbers Authority (IANA) has the worldwide right to assign AS numbers. It delegates that 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, the administrator can use a routing protocol. The routing protocol the administrator uses to connect the networks within the autonomous system is known as an interior gateway routing protocol. Since all networks in an autonomous system belong to the same administrative entity, the administrator can configure any IGP protocol to connect them.
RIPv1, IGRP, OSPF, EIGRP, RIPv2, and IS-IS are some examples of interior gateway routing protocols.
What is an EGP protocol?
An EGP protocol provides connectivity between different autonomous systems. Since different autonomous systems belong to different administrative entities, administrators cannot use routing protocols of their choices to connect them. They have to use a uniform routing protocol. A uniform routing protocol that connects different autonomous systems is known as an exterior gateway routing protocol.
Nowadays, BGP is the only used exterior routing protocol. BGP 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 classifies routing protocols based on their type.
Differences between interior and exterior gateway protocols
The following table compares and lists the difference between Interior and Exterior gateway routing protocols.
| Protocol/ characteristic | RIPv2 | EIGRP | OSPF | IS-IS | BGP |
| Type | Interior | Interior | Interior | Interior | Exterior |
| Sub-type | Distance vector | Hybrid | Link state | Link state | Path vector |
| Metric | Hop count | Bandwidth /delay | Path cost | Path cost | Multiple Attributes |
| Administrative Distance | 120 | Internal 90, external 170 | 110 | 115 | Interior 200, Exterior 20 |
| Hop count limit | 15 | 224 (default 100) | None | None | eBGP -1, iBGP – None |
| Convergency | Slow | Very Fast | Fast | Fast | Average |
| Update | Full Topology/ Periodic | Partial/On change | Partial/On change | Partial/On change | Partial/On change |
RIP
RIP is one of the earliest introduced routing protocols. 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 the triggered updates whenever the metric of the route changes. It has two versions: RIPv1 and RIPv2. RIPv2 was developed as the update of RIPv1.
OSPF
OSPF was developed as 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. It has three versions: OSPFv1, OSPFv2, and OSPFv3. OSPFv2 was developed as the update of OSPFv1. OSPFv3 was developed to support IPv6.
IS-IS
IS-IS is a link-state routing protocol. It was developed as a simple version of OSPF. It is mostly used by ISPs and large internetworks that need a simple and stable routing protocol. It also uses the 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 with incremental updates. 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.
That’s all for this tutorial. In this tutorial, we discussed IGP, EGP, and AS in detail and learned the differences between IGP and EGP.