The idea behind loops is quite simple actually. It seems much more than it is, because often people take it for granted and talk about it as if everyone should know this. :-)
Suppose you have a router A that is connected via a circuit with router Z and there is failure in the circuit. It helps to visualize the circuit between A and Z as 2 arrows. The first arrow starts from Z and points to A, while the second starts from A and points to Z. Along the circuit path there is a number of circuit provider equipment. The circuit is not a looong cable, but consists of many pieces. We are trying to find out which piece is at fault.
We will check the circuit by sending data through it and see if we get the same data back. So, at some point we request a loop along the path. If we are at point A, we request a loop towards A. You can visualize the result as an arrow that starts from A, travels along the circuit path and at the point in the path where the loop is set it changes direction and travels back to reach A.
At the beginning, you could ask to be provided with a loop towards A from their equipment that is closest to Z. If you do not see it, it means that we do not receive the data we send along the path and the circuit is at fault somewhere in the path. This does not exclude the local loop at A. If you confirm that your local loop at A is ok, then the error is in the core of the circuit provider.
When you just check the local loop at A, you request the loop towards A at their equipment that is closest to A. This only checks a small part of the circuit, but if ok, then we move to check the circuit provider core.
Now suppose we requested a loop towards A in the middle of the circuit path. If we see it from A, it means that we send data and receive it from the first half of the circuit, so error is closer to Z (move loop from middle gradually closer to Z until you do not see it from A). If we do not see it, it means that the first half is broken, so error is closer to A (move loop from middle gradually closer to A until you see it from A).
I could probably draw this for you if my drawing skills were not from limited up to non-existent. :-)
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