I’m pretty sure everyone how works on networking stuff had to trace packets, monitor a port on a switch and things like that.
If you need to see what’s going through a specific port on a Cisco switch, the solution is the port spanning.
It allows you to reproduce the traffic of a port on to another one!

I could have done a post describing some basic configuration of a span but quite often, Cisco has a good page on their site to explain in details what it is and how to configure it.

So here is the link to a Cisco article about SPAN.
Switched Port Analyzer

And here the PDF