Transcript ppt in chapter 8

Chapter 8.
Routing Basics
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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 routing is used for taking a packet from one device
and sending it through the network to another device on a
different network.
Routers don’t really care about hosts—they only care about
networks 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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Conti…
If a network isn’t directly connected to the router, the router
must use one of two ways to learn how to get to the remote
network:
 Static routing: meaning that someone must hand-type all
network locations into the routing table.
Dynamic routing: In dynamic routing, a protocol on one router
communicates with the same protocol running on neighbor
routers. The routers then update each other about all the
networks they know about and place this information into the
routing table. If a change occurs in the network, the dynamic
routing protocols automatically inform all routers about the
event.
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Conti..
If static routing is 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 Routing
Static Routing is the process in which the system network
administrator 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 you
must 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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Conti..
The command syntax that is used to add a static route to a routing
table is:
ip route [destination_network] [mask] [next-hop_address or
exitinterface] [administrative_distance] [permanent]
This list describes each command in the string:
ip route The command used to create the static route.
destination_network The network we ’re placing in the routing
table.
mask The subnet mask being used on the network.
next-hop_address The address of the next-hop router that will
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receive the packet and forward it to the remote network.
Conti..
 Exitinterface Used in place of the next-hop address if you want,
and shows up as a directly connected route.
 administrative_distance By 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 the
default value by adding an administrative weight at the end of
the command.
 Permanent If the interface is shut down or the router can’t
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.
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How to Configure Static Routes on Cisco Routers?
To understand the configuration of static routes, consider the
following example:
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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
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RouterA#
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Conti..
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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Conti..
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
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RouterC#
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Conti…
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 route is 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 therefore
less specific.
 We use default routing to 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 networks—those with
only one exit path out of the network.
The syntax for Default routing is :
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Ip route 0.0.0.0 0.0.0.0 <next hop or exit interface
THANK YOU
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