After a loopback test at the CSU/DSU and a good Demarc-to-MDF cable test, where is the fault most likely?

• A. Demarc
• B. Customer Edge Router
• C. MDF
• D. Provider's POP

Answer: (B)

When you can isolate a fault by sequential testing, you start by ruling out the provider side and the main interior copper path. A loopback test at the CSU/DSU that passes shows the path to the provider is good. A good Demarc-to-MDF cable test confirms the cable from the boundary point to the MDF is fine. With both the provider-side path and the primary customer cabling verified good, the remaining likely point of failure is the equipment at the customer edge—specifically the Customer Edge Router. It could be a bad interface, misconfiguration, or a faulty patch to the MDF. The provider’s POP isn’t involved in the fault given the tests passed, and the Demarc and MDF wiring aren’t the issue.