So welcome back, guys.
Now, in this lecture video, we will going to learn how to build pure L3 topology.
Now, when I say pure L3 topology, it means we are talking about a network topology which consists
of only host machines and layer three routers.
There are no L2 switches.
Of course, in the subsequent lectures we will going to build up the topology, which is a mix of L3
routers, host machines and layer two switches.
So this is the sample topology, which is a pure L3 topology that we will going to build.
Now let me give a brief description of this topology.
So in this topology we have a host machine H one and host machine H two which are present at the extreme
opposite ends of this topology.
In between these two hosts, there are three layer three routers which are back to back connected with
each other.
Right?
So this is basically a linear topology.
Now each device in this topology has been assigned a loopback address, right?
So you can see that L3 Routers, R1, R2 and R3 have been assigned a loopback address as well as host
machines also.
And there are four subnets in this topology.
The subnet S1, S2, S3 and S4.
Right.
And we know that every subnet has a network ID, So this table list.
So this table list, the network ID of each subnet in this topology.
So the goal is that we will going to install L3 routes so that every L3 device in this topology is reachable
from every other L3 device.
Right.
And our goal should be that each one host machine should be able to ping s2 host machine and vice versa.
So now that it's a case of pure L3 routing, we need L3 routes to be installed in the Layer three routing
table of each device in this topology, which would make the devices present in different subnet talk
to each other.
So now I will ask you one question.
What are the layer three routes that you need to install in the host machine?
One so that the host machine one can ping any remote device that is present in this topology?
Right?
So if I show you the routing table of the host machine one, the routing table would look something
like this.
So you can see that the routing table of the host machine, H1 would have eight entries, right?
And each entry is actually a route to reach a remote subnet.
Now, the question is that that with respect to the host machine, H1, how many remote subnets are
there in this topology?
So let us count with respect to the host machine, H1.
S1 is a local subnet, right?
So it is not a remote subnet.
The subnet S2 is a remote subnet, right.
And so as the subnet S3 and the subnet S4 right now, the loopback address of the router R1 is also
a remote subnet with respect to the host machine, H1.
And so the loopback address of the router, R2 loopback address of the router, R3 and loopback address
of the host machine S2.
So if you count all of these, it means that there are seven remote subnets with respect to the host
machine H1 in this topology and therefore you need to tell the host machine H1 how to reach each of
these seven remote subnets of the topology.
And finally, we have a default route installed in the routing table of the host machine.
That's one, right?
So I have shown you the sample routing table of the host machine Edge one.
We will install similar routes in the routing table of each device in this topology, such that every
device in the topology has a route to reach every other device in this topology.
So in the next lecture video, let us build up this pure L3 routing topology and let's see the things
in action.