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Before doing a demonstration a quick recap of a routing table, what is a routing table? A routing table

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is a list of networks that the router knows about as well as information about how to reach those networks.

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In other words, a router may have a network 10.1.1.1/24 in the routing table

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that entry may also contain information such as the outgoing local interface and the next top router

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to use to get to that destination.

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Each entry in the routing table is a destination network

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if you're using a unicast routing protocol and once again includes the local interface to use

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to get to the neighbor or the next hop to get to that destination.

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Now there are various types of routes that are included in a routing table including directly connected

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networks.

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These networks are added to the writing table automatically and is a network that is directly connected

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to one of the interfaces on the local router.

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If the interface goes down well in other words fails or you shut down the interface.

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In other words, it's administratively down that network is removed from the routing table.

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The second type is static routes.

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Static routes are routes that you as the administrator have manually added to the configuration of the

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router.

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Static routes are very good for small simple networks that don't change often.

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However, if your networks very large static routes don't work well because of the amount of work it is

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to maintain those routing entries. Static routes also do not dynamically adjust to changes in the network

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topology.

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So if you're telling a router to go right to get to a destination and the network is down the router

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will still send traffic to the right rather than going to the left as an example to get to the destination.

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The third type is a default route.

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You can manually configure a default route.

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It's a special type of static route or a routing protocol can dynamically add it to the routing table

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a default route is an optional route that is used when no explicit path to a destination is found in

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the routing table.

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So if a specific route like 10.1.10/24 is not in the routing table the default route can be

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used to get to the destination.

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In this topology, I have three routers, router 1, router 2, and router 3. I've configured loopbacks on each

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router.

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So quadruple 1 on router 1, quadruple 2 on router 2, quadruple 3 on router 3. Fast

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Ethernet00 on router 1 and router 2 is in the 10.1.1.0/24 network

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f0/1 on router 2 and f0/0 on router 3 are in the 10.1.2.0/24 network. Here's router 1 show

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IP interface brief.

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I can see that IP address 10.1.1.

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1 is configured on FastEthernet 00 and 1.1.1.1 is configured on loopback 0.

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Show IP route in the output we can see that two directly connected networks have been added to the writing

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table of Rato one 10 1 1 0 slushed 24 is directly connected to Fost.

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Ethan it is 0 0.

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Its important to remember that routers and layer 3 switches route based on networks and subnets and

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not based on IP addresses show run interface Fost.

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Ethan It is 00 shows me that the IP address configured on FASA Ethan it is 0 0 is 10 that wandered wondered

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1 slash 24 but the IP address is not added to the writing table.

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The network is added to the writing table as a directly connected network on fust.

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Ethan it is 0 0 and its shown in the writing table we're denoted in the writing table as a C which means

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connected network.

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When you add an IP address to an interface and that interface is up the road has added to the writing

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table.

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So let's prove that interface loop back one IP address actually before I put the IP address on let's

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have a look at the writing table do show IP ront.

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Notice we only see two metrics in the writing table.

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However if I configure an additional network so $1.1 1 to slash 32 on this new loopback and now use

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the command to show IP route notice we have a third route in the writing table.

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We have 1.1 1.1 slashed 32 and 1.1 one to slash 32 this network is configured to loop back zero.

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And this network is configured on loopback back one.

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The reason these display as IP addresses is because we configure it a slash 32 mosque.

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However here we configure configured to slash 24 mosque.

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So we see tenders one but 1.0 So as another example interface loop back to IP Address 10 dot one and

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let's make this 10 one slice 24.

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What does that add to our writing table do show IP route notice slash 24 network is added to the writing

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table because we used a slash 24 mosque on the site.

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The address if you need to face is shut down.

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The rod is removed from the writing table.

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So this route is currently in the writing table.

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I'll shut it down to show IP route notice the network is removed because the interface has gone down.

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No shut it interface comes up do you show IP route network is now available.

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However at the moment we do not have rats in the writing table for Rodda two and three.

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So lets add those ranched to the writing table.