What is the main purpose of Routing?

A question that I always get from students, whether from my online video courses or from classroom students: “Why we need routing in our network?”

It is a very smart question from students who don’t know yet the importance of routing protocols.

To answer the question, I always explain what if we do not have routing protocols in our networks, then what would happen?

Say that you have a complete layer 2 network, meaning that all end devices are connected to each other via switches (so there are no Layer 3 devices such as routers).

Let’s assume that the internet is connected via Layer 2 (which is impossible but just let’s assume that). In this case, if a PC located in London send a broadcast traffic, then all end devices in the whole world will be received this broadcast traffic, and guess what: IT IS INFINITE. That means that the broadcast traffic will never end because there is not any mechanism on Layer 2 to kill the traffic such as Time to Live (TTL) which is on Layer 3 (we will speak later about TTL and do a LAB about it in this course).

In a summary, having a flat Layer 2 network is impossible and we won’t have the possibility to be connected to the internet in case using Layer 2 devices only.

You may use a Layer 2 network if can you have a closed network (not connected to the internet), but whenever you need to be connected to the internet or to another network then you need a Layer 3 device such as a router on which you can do the routing.

I hope I could clarify the idea of the essence of having routing in our network.

Now while speaking about routing, it is important to know that we do have many routing protocols, and these can be used either inside or outside the network.

Let me show you this using the following illustration:

In the illustration, there are 2 different Autonomous Systems (AS) which are:

AS 100
AS 200

In each AS, there are many routers connected to each other. For now, we know that a router is a Layer 3 device, meaning that its main function it to make routing. But what is exactly an AS?

By definition, an AS is a set of routers which are under the privilege and administration of a network engineer. In other words, you can say that whenever you have a company with many routers connected to each other’s within the same network, and you have access to all those routers in that company, then this is called an AS.