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Let me suggest another approach. What NOS are you using? Netware and Windows both allow you to create groups and assign rights. In this case you would create a group that is allowed to print to the printer in question and assign only the 2 users. ...
When you loopback the interface through the CLI it is looped back in the hardware of the router. This is not the same as doing it at the CSU/DSU or beyond. When you loopback using the CLI you are not testing the serial cable, CSU/DSU, carrier line,...
No sure answer here - but some things to consider. When the CSU/DSU is able to detect carrier form the remote end it will raise DCD. If there is a fault between you and your carrier on your receive, the carrier won't detect it but you will (the tro...
In order to ping or telnet or do anything else for that matter, you need a route to the destination and the destination needs a route back. Looking at your configurations, I can't tell if you have provided these routes. I do see a couple of things ...