On a Cisco router, which command will show you whether a DTE or DCE cable is connected to serial interface 0 by displaying detailed hardware controller information?

Introduction / Context:
This question addresses low level hardware diagnostics on Cisco routers, specifically the ability to determine whether a serial interface is connected using a Data Communication Equipment or Data Terminal Equipment cable. This is particularly relevant in lab environments where one router often provides clocking to another and you must know which end is DCE.

Given Data / Assumptions:
- The interface in question is serial 0, often written as s0 or serial 0 in commands.
- You want to know whether the connected cable is considered DTE or DCE from the router's perspective.
- Standard show interface commands provide useful status but may not include this specific hardware level detail.

Concept / Approach:
On Cisco devices, the show controllers command displays internal diagnostic information about interfaces, including whether a synchronous serial interface sees a DTE or DCE cable. For example, show controllers s0 may output a line stating cable type DTE or cable type DCE. This is more detailed than the general show interface output, which focuses on line status, encapsulation, and statistics, but not necessarily cable type.

Step-by-Step Solution:
Step 1: Recall that DCE devices provide clocking on synchronous serial links, and DTE devices receive that clock.Step 2: In lab setups, one router often plays the role of DCE, so you need to know which end is which to configure clock rate correctly.Step 3: Recognize that show interface serial 0 provides information such as IP address, line protocol status, and encapsulation, but not necessarily cable type.Step 4: Remember that show controllers s0 accesses hardware specific details, including DCE or DTE designation.Step 5: Select show controllers s0 as the correct command.

Verification / Alternative check:
In a lab, you can run show controllers s0 on each router connected by a back to back serial cable. One side will typically report that it has a DCE cable, while the other reports a DTE cable. The router with the DCE cable is the one where you must set the clock rate on the interface. This real world behavior corroborates the purpose of the show controllers command.

Why Other Options Are Wrong:
show interface s0 and show interface serial 0 provide general interface status but do not always include cable type details. The command show serial 0 controllers does not match standard IOS syntax. show ip interface s0 focuses on IP related details and again does not display physical cable type. Only show controllers s0 is explicitly designed to provide the low level hardware information that includes DTE or DCE designation.

Common Pitfalls:
Many learners default to show interface for all diagnostics and forget that some hardware specific information is available only through show controllers. Another pitfall is confusing DTE and DCE roles and not realizing that only the DCE end can provide clocking on a lab link. Memorizing the association between show controllers and serial DCE/DTE information helps resolve these issues quickly in exams and real configurations.

Final Answer:
The command show controllers s0 displays controller details, including whether a DTE or DCE cable is connected to serial 0.