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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, etc. When an interface comes up and runs clean to a loop, it verifies all equipment to the loop point. This is a good troubleshooting tool. Move your loop point in either direction until you isolate the faulty part.
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