Difficulty: Easy
Correct Answer: Physical layer
Explanation:
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:
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:
Common Pitfalls:
Final Answer:
Physical layer
Discussion & Comments