What is a Loopback Interface, and when would we use one?

Scenario:

Let’s say we have a network topology like the diagram below:

Loopback 2
If an administrator connected to R1 tries to telnet/SSH onto the fa0/1 interface on R5, but for some reason the interface is down or not stable, then what will happen?

He won’t be able to connect!

The workaround? To try and connect to one of the other interfaces of the Router to gain access. So he tries interface Serial 0/1/0…….But even if that works, it took 2 tries to successfully connect.

It can be bothersome, especially when the number of links to a device increase, the number of tries will increase likewise for every other interface that may be down

Also, workarounds are only temporary in nature, we want something a bit more permanent.

The solution for this is to have what is known as a Loopback interface on the Router.

A Loopback interface is an interface which is completely virtual, it doesn’t exist in real life.

It’s a virtual interface which is always ‘up’ and on. It isnt subject to the same weaknesses and points of failure as a physical interface is.

It can be configured just like a normal interface which means we can assign an IP address to that interface.

Now how does this help us?

Instead of trying to telnet/SSH into the Router using one of the physical interfaces, we can go into it via the loopback interface!

Since the IP of this interface is being shared via any existing routing protocols, all the Routers know of this interface….. and the interface will always be up!

We now have a constant, always-up connection to our device which we can connect to in one go…awesome!

This scenario is but one of many which I hope will illustrate the usefulness of Loopback Interfaces.

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