Before doing a demonstration a quick recap of a routing table, what is a routing table? A routing table
is a list of networks that the router knows about as well as information about how to reach those networks.
In other words, a router may have a network 10.1.1.1/24 in the routing table
that entry may also contain information such as the outgoing local interface and the next top router
to use to get to that destination.
Each entry in the routing table is a destination network
if you're using a unicast routing protocol and once again includes the local interface to use
to get to the neighbor or the next hop to get to that destination.
Now there are various types of routes that are included in a routing table including directly connected
networks.
These networks are added to the writing table automatically and is a network that is directly connected
to one of the interfaces on the local router.
If the interface goes down well in other words fails or you shut down the interface.
In other words, it's administratively down that network is removed from the routing table.
The second type is static routes.
Static routes are routes that you as the administrator have manually added to the configuration of the
router.
Static routes are very good for small simple networks that don't change often.
However, if your networks very large static routes don't work well because of the amount of work it is
to maintain those routing entries. Static routes also do not dynamically adjust to changes in the network
topology.
So if you're telling a router to go right to get to a destination and the network is down the router
will still send traffic to the right rather than going to the left as an example to get to the destination.
The third type is a default route.
You can manually configure a default route.
It's a special type of static route or a routing protocol can dynamically add it to the routing table
a default route is an optional route that is used when no explicit path to a destination is found in
the routing table.
So if a specific route like 10.1.10/24 is not in the routing table the default route can be
used to get to the destination.
In this topology, I have three routers, router 1, router 2, and router 3. I've configured loopbacks on each
router.
So quadruple 1 on router 1, quadruple 2 on router 2, quadruple 3 on router 3. Fast
Ethernet00 on router 1 and router 2 is in the 10.1.1.0/24 network
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
IP interface brief.
I can see that IP address 10.1.1.
1 is configured on FastEthernet 00 and 1.1.1.1 is configured on loopback 0.
Show IP route in the output we can see that two directly connected networks have been added to the writing
table of Rato one 10 1 1 0 slushed 24 is directly connected to Fost.
Ethan it is 0 0.
Its important to remember that routers and layer 3 switches route based on networks and subnets and
not based on IP addresses show run interface Fost.
Ethan It is 00 shows me that the IP address configured on FASA Ethan it is 0 0 is 10 that wandered wondered
1 slash 24 but the IP address is not added to the writing table.
The network is added to the writing table as a directly connected network on fust.
Ethan it is 0 0 and its shown in the writing table we're denoted in the writing table as a C which means
connected network.
When you add an IP address to an interface and that interface is up the road has added to the writing
table.
So let's prove that interface loop back one IP address actually before I put the IP address on let's
have a look at the writing table do show IP ront.
Notice we only see two metrics in the writing table.
However if I configure an additional network so $1.1 1 to slash 32 on this new loopback and now use
the command to show IP route notice we have a third route in the writing table.
We have 1.1 1.1 slashed 32 and 1.1 one to slash 32 this network is configured to loop back zero.
And this network is configured on loopback back one.
The reason these display as IP addresses is because we configure it a slash 32 mosque.
However here we configure configured to slash 24 mosque.
So we see tenders one but 1.0 So as another example interface loop back to IP Address 10 dot one and
let's make this 10 one slice 24.
What does that add to our writing table do show IP route notice slash 24 network is added to the writing
table because we used a slash 24 mosque on the site.
The address if you need to face is shut down.
The rod is removed from the writing table.
So this route is currently in the writing table.
I'll shut it down to show IP route notice the network is removed because the interface has gone down.
No shut it interface comes up do you show IP route network is now available.
However at the moment we do not have rats in the writing table for Rodda two and three.
So lets add those ranched to the writing table.