So, guys, in this slide, I will show you or explain the three basic steps which every L3 device in
the topology follows or implement in order to finally compute its local routing table.
So those three steps are built.
Input graph.
Apply the algorithm on this graph and compute the output.
In this case, the output is layer three routing table.
So you can see in this diagram, suppose we have a physical network topology consisting of routing devices
connected via links or wires.
So this is a physical topology.
Now the very first step that is involved is to how to build the input graph data structure.
Each router, which is present in this topology, needs to build a graph data structure in their memory,
and that particular graph represents this exact physical topology.
So these blue colored devices are just routers, which are the part of this topology.
So to illustrate an example, I have taken the devices A, B and C, but assume that whatever explanation
I provide or give also applies to rest of the devices which I have not shown on this slide.
For example, on the device S and D, So we need to do something such that each device which is present
in this topology builds an input graph data structure in its local memory.
And that graph data structure actually should represent this physical topology as it is.
This graph data structure must have all the details which is present in this physical topology, including
how the different nodes are connected to each other.
What is the IP addresses configured on interfaces of the nodes and what are the link cost or name of
the interfaces or any other detail?
So you must have some routing protocol process be running on each device on this topology.
So this process be running on each of this device is supposed to build a data structure graph, which
actually is a mirror of this physical topology.
We will discuss in the next section of this course that how this is actually achieved.
Then the second step that each routing device does is to apply the algorithm on its input graph data
structure, which they have built in their local memory.
Right.
Now, this input algorithm is a networking oriented algorithm, meaning that this algorithm applies
on the graph, which represents network topology.
And the output of this algorithm is something which is used in the networking.
In this case, the output is nothing, but it is an L3 routing table.
So the output of applying this algorithm on a.
Local graph data structure which device has built in its local memory is the routing table.
Right?
Networking is all about applying algorithms on the graph data structure because network topology is
naturally a graph data structure, right?
We apply various algorithms on this graph data structures to compute various network oriented results
such as routing table and so on.
So in the next section of this course, we will going to break down each of these steps in more detail
and see how L3 routing device accomplish each of these steps to finally build or compute its local routing
table.