Eduardo
Without knowing a bit more about what you intend to do it is difficult to give a really good answer to your question. If the new circuit is connected to a live router (I assume that this means that the router has other active interfaces and is forwarding traffic for these interfaces?) you should be able to do no shutdown on the interface and see if it comes up with no impact on the router processing. That would be the first test that I would do.
You could then do another test and enable CDP on the interface (assuming that there is another Cisco router on the other end of the circuit) and look to see if you learn about the neighbor on the other end of the circuit. This would test the circuit and not impact existing traffic.
If you want to go beyond that and assign an IP address to the new circuit interface and attempt to send IP over the new circuit it should be possible to do this without impact to the existing traffic. But you do need to be careful about addressing, subnetting, and how you are routing existing traffic and routing traffic on the new circuit and a mistake in this could impact existing traffic.
HTH
Rick
HTH
Rick
