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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)#ip route 193.168.2.0 255.255.255.0 10.1.1.2
(define static routing on Router A)
RouterA(config)#ip route 193.168.3.0 255.255.255.0 20.1.1.1
(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)#ip route 193.168.1.0 255.255.255.0 10.1.1.1
(define static routing on Router B)
RouterB(config)#ip route 193.168.3.0 255.255.255.0 15.1.1.2
(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)#ip route 193.168.2.0 255.255.255.0 15.1.1.1
(define static routing on Router C)
RouterC(config)#ip route 193.168.1.0 255.255.255.0 20.1.1.2
(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