Layer diagnosis from interface status You issue show interface serial 1 and see: “Serial1 is down, line protocol is down.” Which OSI layer is most likely where the fault resides?

Introduction / Context:
Cisco IOS provides two key indicators per interface: the physical interface status and the line protocol status. These map closely to OSI Layer 1 (Physical) and Layer 2 (Data Link). Being able to infer the likely fault domain from the combined status accelerates troubleshooting of WAN serial links and Ethernet ports alike.

Given Data / Assumptions:

• Status shown is “down/down” (both the interface and the line protocol are down).
• No “administratively” qualifier is present.
• We are working with standard encapsulations and defaults.

Concept / Approach:

“Down/down” often indicates a Layer 1 problem: bad or disconnected cable, wrong CSU/DSU settings, no clocking on DCE, or hardware failure. When Layer 1 is down, the Layer 2 protocol cannot come up, hence both states are down. By contrast, “up/down” points to a Layer 2 mismatch (encapsulation, keepalives), and “administratively down” signals a configured shutdown.

Step-by-Step Solution:

Verification / Alternative check:

Use show controllers serial to verify DTE/DCE state and clocking, check cabling, and consult CSU/DSU LEDs to confirm signal presence.

Why Other Options Are Wrong:

• Data Link or Network layers: would typically present as “up/down” or routing issues with “up/up.”
• Router software problem: not indicated by this specific status pair.
• Transport layer: unrelated to interface link states.

Common Pitfalls:

• Jumping to reconfigure encapsulation when the cable is simply unplugged.
• Ignoring DCE/DTE roles and missing clock rate on lab serial connections.

Final Answer:

Physical layer