Complete Guide to Network Routing Protocols: Everything About IGP and EGP

NetworkRoutingProtocolIGPEGPOSIBGPOSPFRIPAutonomous SystemNetwork SecurityInternet

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Published: 2025-06-26

Last modified: 2025-06-26

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Summary

A systematic overview of routing protocols in the network layer. This guide covers the differences between static and dynamic routing, the distinction between Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP), and detailed characteristics and algorithms of RIP, OSPF, and BGP based on the concept of Autonomous Systems (AS).

Starting from the network layer (OSI layer 3)
Routing protocols are classified into static routing and dynamic routing
Dynamic routing is further divided into Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP)
IGP includes RIP and OSPF, while EGP includes BGP

Keywords

Routing

Routing refers to the process of establishing paths for data transmission in the network layer.

Static Routing vs Dynamic Routing

Static Routing - A method that predefines paths during router configuration and uses fixed routes.
Dynamic Routing - A method that automatically calculates paths during router configuration and dynamically changes routes.

Interior Gateway Protocol (IGP) vs Exterior Gateway Protocol (EGP)

Interior Gateway Protocol (IGP) - Routing protocol used within the same network.
Exterior Gateway Protocol (EGP) - Routing protocol used between different networks.

Criteria for Distinguishing Same Network vs Different Networks

Autonomous System (AS)

The most important distinction criterion is the Autonomous System (AS).

Same Network (Same AS): Network domain managed by a single organization or institution
Different Network (Different AS): Network domains managed by different organizations or institutions

Management Authority Criteria

Same Network: Domain controlled by the same network administrator or organization
Different Network: Networks operated by different management entities

Specific Examples

Same Network (IGP Usage)

Multiple departmental networks within a company
Networks of different buildings within a university campus

Different Networks (EGP Usage)

Networks of different telecommunications companies like KT and SKT
Internet connections between Korea and the United States
Connections between different ISPs (Internet Service Providers)

Distinction by AS Numbers

Each autonomous system has a unique AS number:

AS 1 ~ AS 65535: Public AS numbers
Examples: KT (AS 4766), SKT (AS 9318), Google (AS 15169)

Interior Gateway Protocol (IGP)

RIP (Routing Information Protocol)

Uses Distance Vector Algorithm - Bellman-Ford Algorithm
Each router receives distance information from neighboring routers to update routing table
Supports up to 15 hops based on hop count

OSPF (Open Shortest Path First)

Uses Link State Algorithm - Dijkstra Algorithm
Each router calculates shortest path after understanding entire network topology
Suitable for large-scale networks with fast convergence characteristics

Exterior Gateway Protocol (EGP)

BGP (Border Gateway Protocol)

Uses Path Vector Algorithm
Exchanges complete AS path information to destinations
Core protocol used in internet backbone

Routing Algorithm Comparison

Distance Vector vs Link State vs Path Vector

Category	Distance Vector	Link State	Path Vector
Representative Protocol	RIP	OSPF	BGP
Information Exchanged	Distance (hop count)	Complete topology	Complete path
Algorithm	Bellman-Ford	Dijkstra	Path Vector
Usage Scope	Small networks	Large networks	Inter-AS connections