Ch 8 Routing Basics

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Routing Basics

Chapter 8.


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CHAPTER OBJECTIVES

At the end of this Chapter you will be able to:

Understand routing basics.


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Routing Fundamentals

Introducing Routing

The term rout ingis used for taking a packet from one device

and sending it through the network to another device on a

different network.

Routers dont really care about hoststhey only care aboutnetworks and the best path to each network.

The logical network address of the destination host is used to

get packets to a network through a routed network,

hardware address of the host is used to deliver the packet from

a router to the correct destination host.


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Conti

To be able to route packets, a router must know, at a minimum, the

following:

Destination address.

Neighbor routers from which it can learn about remote networks.

Possible routes to all remote networks.

The best route to each remote network.

How to maintain and verify routing information.

The router learns about remote networks from neighbor routers or

from an administrator.


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Ifstat ic rout ingis used, the administrator is responsible for

updating all changes by hand into all routers. Typically, in a large

network, a combination of both dynamic and static routing is used

IP Routing

What is IP Routing?

As it is already discussed that IP routing is basically of two types:

Static routing

Dynamic routing


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Static RoutingStat ic Routin gis the process in which the system networkadministrator would manually configure network routers with all

the information necessary for successful packet forwarding. The

administrator constructs the routing table in every router by

putting in the entries for every network that could be a

destination.

Advantages:

Static routes are simple and quick to configure.

Static routing is supported on all routing devices and all

routers.

Static routes are easy to predict and understand in small

networks


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Conti..

Disadvantages:

Static routes require extensive planning and have high

management overhead. The more routers exist in a network, the

more routes that need to be configured. If you have 'N' number of

routers and a route between each router is needed, then youmust configure N x N routes, so, for a network of nine routers,

you need 81 routes (9 x 9 = 81).

Static routes do not dynamically adapt to network topology

changes or equipment failures.

Static routing does not scale well in large networks.


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The command syntaxthat is used to add a static route to a routingtable is:

ip route [dest inat ion_netwo rk] [mask ] [next-hop_address or

exit interface] [admin istrat ive_distance] [p ermanent]

This list describes each command in the string:

ip routeThe command used to create the static route.

dest inat ion_networkThe network we re placing in the routing

table.

maskThe subnet mask being used on the network.

next-hop_addressThe address of the next-hop router that will

receive the packet and forward it to the remote network.

Conti..


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Exit inter faceUsed in place of the next-hop address if you want,and shows up as a directly connected route.

administrat ive_distanceBy default, static routes have an

administrative distance of 1 (or even 0 if you use an exit

interface instead of a next-hop address). You can change thedefault value by adding an administrative weight at the end of

the command.

PermanentIf the interface is shut down or the router cant

communicate to the next-hop router, the route will automatically

be discarded from the routing table. Choosing the permanent

option keeps the entry in the routing table no matter what

happens.

Conti..


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How to Configure Static Routes on Cisco Routers?

To understand the configuration of static routes, consider the

following example:

Configuration of Static Routes


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Commands

To configure static routes, first enter global configuration mode to

run the following commands.

Configure the Static Routes on Router A.

RouterA#configure terminal (enter in global configuration mode)

RouterA(config)#ip route 15.0.0.0 255.0.0.0 10.1.1.2

(define static routing on Router A)





RouterA(config)#exit

RouterA#


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Configure the Static Routes on Router B.RouterB#configure terminal (enter in global configuration mode)

RouterB(config)#ip route 20.0.0.0 255.0.0.0 10.1.1.1

(define static routing on Router B)





RouterB(config)#exit

RouterB#


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Configure the Static Routes on Router C.RouterC#configure terminal (enter in global configuration mode)

RouterC(config)#ip route 10.0.0.0 255.0.0.0 15.1.1.1

(define static routing on Router C)





RouterC(config)#exit

RouterC#


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Now run the command show ip route on every router to view the IP

routing table (directly connected + static routes) detail.

2. Default Route

A default routeis often called the route of last resort'. It is the

last route tried by a router when all other routes fail because it

has the fewest number of network bits matching and is thereforeless specific.

We use default rout in gto send packets with a remote destination

network not in the routing table to the next-hop router. You

should only use default routing on stub networksthose with

only one exit path out of the network.

The syntax for Default routing is :

Ip route 0.0.0.0 0.0.0.0


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THANK YOU

Ch 8 Routing Basics

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