Chapter 10

Routing Protocols

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This chapter presents the general principles of router functioning and the different types of routing protocols.

Chapter 11 presents the internal routing protocols RIP, OSPF and IS-IS.

Chapter 12 presents the external routing protocol BGP.

10.1. Routing tables

The IP protocol and addressing space allocation by a single organism have allowed the creation of a worldwide covering network. For a packet to reach its destination, it must go through routers that transport it. A router is a device that has several interfaces1. It contains tables determining the output interface based on the packet’s destination address.

An address has several functions in the Internet:

— Localization: i.e. being able to find the location of a device in the network. This function is of interest for the routing of packets;

— Identification: i.e. designates a unique device. Identification is used by TCP to locate a connection.

A routing table, also called a FIB (forwarding information base), can be summarized by two kinds of information: to go towards the correct destination, packets must exit by such interface.

— The destination can either be an IP network or subnetwork (i.e. a set of devices sharing a common address part), or a particular device. In the case of a device, the complete IP address is given. For an IP network, bits reserved for the device number are set to 0. The ...

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