Routing Basics Chapter 8. powered by DJ 1

C HAPTER O BJECTIVES At the end of this Chapter you will be able to:  Understand routing basics. powered by DJ 2

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. powered by DJ 3

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. powered by DJ 4

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. powered by DJ 5

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 powered by DJ 6

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 powered by DJ 7

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. powered by DJ 8

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 receive the packet and forward it to the remote network. Conti.. powered by DJ 9

 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. Conti.. powered by DJ 10

How to Configure Static Routes on Cisco Routers? To understand the configuration of static routes, consider the following example: Configuration of Static Routes powered by DJ 11

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 (define static routing on Router A) RouterA(config)#ip route (define static routing on Router A) RouterA(config)#ip route (define static routing on Router A) RouterA(config)#exit RouterA# powered by DJ 12

Conti.. Configure the Static Routes on Router B. RouterB#configure terminal (enter in global configuration mode) RouterB(config)#ip route (define static routing on Router B) RouterB(config)#ip route (define static routing on Router B) RouterB(config)#ip route (define static routing on Router B) RouterB(config)#exit RouterB# powered by DJ 13

Conti.. Configure the Static Routes on Router C. RouterC#configure terminal (enter in global configuration mode) RouterC(config)#ip route (define static routing on Router C) RouterC(config)#ip route (define static routing on Router C) RouterC(config)#ip route (define static routing on Router C) RouterC(config)#exit RouterC# powered by DJ 14

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 : Ip route <next hop or exit interface powered by DJ 15

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