If you type show interface serial 1 on a Cisco router and see Serial 1 is down, line protocol is down, at which OSI layer would you first suspect a problem?

Introduction / Context:
Interface status messages on Cisco routers provide important clues about where a problem exists in the OSI model. The show interfaces command displays both the physical state of the interface and the line protocol state. Learning how to interpret messages such as is down, line protocol is down is essential for troubleshooting WAN links and serial connections.

Given Data / Assumptions:

• The command show interface serial 1 was executed on a router.
• The output shows Serial 1 is down, line protocol is down.
• The interface is a serial interface, typically used for WAN connections.
• No additional configuration information is provided, so we focus on interpreting the overall status line.

Concept / Approach:
On Cisco devices, the first part of the status line (is up or is down) reflects the physical layer status. It indicates whether the interface detects carrier and whether the hardware is administratively enabled. The second part (line protocol is up or down) reflects data link layer status, such as keepalive messages and encapsulation. When both the interface and the line protocol are shown as down, this usually points to a physical layer problem, such as a bad cable, no clocking, or an unplugged connection. A data link layer problem typically shows up as is up, line protocol is down.

Step-by-Step Solution:
Step 1: Interpret Serial 1 is down as indicating that the physical interface is not operational, either because it is administratively shut down or because there is no carrier detection.Step 2: Interpret line protocol is down as indicating that the data link encapsulation is also not operational, which is expected when the physical layer is down.Step 3: Recognize that when both states are down, the root cause is most often at the physical layer.Step 4: Consider common physical problems such as disconnected cables, faulty CSU or DSU, wrong clocking, or a shut interface.Step 5: Conclude that you should begin troubleshooting at the OSI Physical layer before investigating higher layer issues.

Verification / Alternative check:
You can verify this interpretation by checking cabling, LED indicators on CSU or DSU devices, and by using the no shutdown command on the serial interface. If the physical problem is resolved, the interface status will change to is up and the line protocol may then come up as well, confirming that the fault was at Layer 1.

Why Other Options Are Wrong:
Option B corresponds more to cases where the interface is up but the line protocol is down, which usually indicates a Layer 2 problem.Option C refers to Layer 3 routing issues, which would not normally cause both the interface and line protocol to show as down.Option D and Option E involve higher layer functions that depend on a working physical and data link layer and are not the first suspects here.

Common Pitfalls:
New engineers sometimes jump immediately to routing or configuration issues when they see connectivity problems. A structured approach starts at the bottom of the OSI model. When both the interface and line protocol are down, always suspect physical layer issues first before looking at Layer 2 or Layer 3 settings.

Final Answer:
You should first suspect a problem at the Physical layer (Layer 1) when you see Serial 1 is down, line protocol is down.