Routing protocols

Routing protocols

6.3.3

This page will give some examples of routing protocols and how they are used.
At the Internet layer of the TCP/IP suite of protocols, a router can use an IP routing protocol to accomplish routing through the implementation of a specific routing algorithm. Examples of IP routing protocols include the following:

• RIP - a distance vector interior routing protocol
• IGRP - the Cisco distance vector interior routing protocol
• OSPF - a link-state interior routing protocol
• EIGRP - the advanced Cisco distance vector interior routing protocol
• BGP - a distance vector exterior routing protocol

RIP was originally specified in RFC 1058. Its key characteristics include the following:

• It is a distance vector routing protocol.
• Hop count is used as the metric for path selection.
• If the hop count is greater than 15, the packet is discarded.
• Routing updates are broadcast every 30 seconds, by default.

IGRP is a proprietary protocol developed by Cisco. Some of the IGRP key design characteristics are as follows:

• It is a distance vector routing protocol.
• Bandwidth, load, delay and reliability are used to create a composite metric.
• Routing updates are broadcast every 90 seconds, by default.

OSPF is a nonproprietary link-state routing protocol.

• It is a link-state routing protocol.
• It is an open standard routing protocol described in RFC 2328.
• The SPF algorithm is used to calculate the lowest cost to a destination.
• Routing updates are flooded as topology changes occur.

EIGRP is a Cisco proprietary enhanced distance vector routing protocol. The key characteristics of EIGRP are as follows:

• It is an enhanced distance vector routing protocol.
• It uses unequal cost load balancing.
• It uses a combination of distance vector and link-state features.
• It uses Diffused Update Algorithm (DUAL) to calculate the shortest path.
• Routing updates are multicast using 224.0.0.10 triggered by topology changes.

Border Gateway Protocol (BGP) is an exterior routing protocol. The key characteristics of BGP are as follows:

• It is a distance vector exterior routing protocol.
• It is used between ISPs or ISPs and clients.
• It is used to route Internet traffic between autonomous systems.

The next page will discuss interior and exterior routing protocols.

Routing configuration

Routing configuration 

6.3.2 
This page will explain the steps that are used to configure a routing protocol.
To enable an IP routing protocol on a router, global and routing parameters need to be set. Global tasks include the selection of a routing protocol such as RIP, IGRP, EIGRP, or OSPF. The major task in the routing configuration mode is to indicate IP network numbers. Dynamic routing uses broadcasts and multicasts to communicate with other routers.
The router command starts a routing process.  
The network command enables the routing process to determine which interfaces send and receive routing updates.  
An example of a routing configuration is as follows:

GAD(config)#router rip

GAD(config-router)#network 172.16.0.0

For RIP and IGRP, the network numbers are based on the network class addresses, not subnet addresses or individual host addresses.
The Lab Activity will help students configure routers to start a routing process.
The next page will describe some routing protocols.

Routing Protocols Overview / Path determination

Routing Protocols Overview

Path determination 

6.3.1
This page will explain how a router determines the path of a packet from one data link to another. The router uses two basic functions:

• A path determination function
• A switching function

Path determination occurs at the network layer. The path determination function enables a router to evaluate the paths to a destination and to establish the preferred way to handle a packet. The router uses the routing table to determine the best path and then uses the switching function to forward the packet. -
The switching function is the internal process used by a router to accept a packet on one interface and forward it to a second interface on the same router. A key responsibility of the switching function of the router is to encapsulate packets in the appropriate frame type for the next data link.
Figure illustrates how routers use addressing for these routing and switching functions. The router uses the network portion of the address to make path selections to pass the packet to the next router along the path.
The next page will describe the commands that are used to configure a routing protocol.