CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
R1# show controllers Serial0/0/0 Interface Serial0/0/0 Hardware is PowerQUICC MPC860 DCE V.35, no clock rate idb at 0x81081F80, driver data structure at 0x81086768 scc registers: rx ring entries = 64, tx ring entries = 64 rx ring start = 0x405F20, tx ring start = 0x405F60 buffer size 1524, control block pointer 0x81B00C cable type : V.35 DCE cable no clock rate configured transmitter delay is 0 microseconds
Refer to the exhibit. A network engineer is troubleshooting a serial link between two routers that is not coming up. The engineer issues the show controllers command on one router and sees the output shown. What is the most likely cause of the issue?
⚠ Common exam trap
Cisco often tests the distinction between DCE and DTE roles; the trap here is that candidates assume the 'show controllers' output is irrelevant or that the issue is a Layer 2 problem (encapsulation) when the root cause is a missing Layer 1 clock signal on the DCE side.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The clock rate command is missing on the DCE serial interface.
The 'show controllers' output indicates the router is the DCE (Data Communications Equipment) on the serial link, but no clock rate has been configured. For a serial interface to come up, the DCE end must provide clocking via the 'clock rate' command. Without it, the interface will remain down (line protocol down) because no clock signal is present to synchronize data transmission.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The clock rate command is missing on the DCE serial interface.
Why this is correct
The show controllers output clearly states “DCE V.35, no clock rate” and “no clock rate configured,” confirming that the DCE side lacks the required clock rate. Without it, the serial interface cannot bring the line protocol up.
- ✗
The serial cable type is incorrectly identified.
Why it's wrong here
The show controllers output explicitly lists 'DCE V.35, no clock rate,' meaning the router has correctly identified the cable as a V.35 DCE crossover cable. If the cable type were misidentified, the output would typically display an unrecognized or incorrect interface type rather than clearly naming V.35 DCE. Therefore, the cable identification is accurate and is not the cause of the line protocol being down.
- ✗
The encapsulation mismatch is causing the line protocol to stay down.
Why it's wrong here
Encapsulation type is not shown in the exhibit, and the down state is directly linked to the missing clock rate on the DCE side. An encapsulation mismatch would typically keep the line protocol down but would not be the root cause given the current output.
- ✗
The serial interface is administratively shut down.
Why it's wrong here
If the interface were administratively shut down, the show controllers output would not display the detailed hardware information as active. The exhibit makes no mention of an administratively down state; it shows the interface detecting the cable and noting the missing clock rate.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The clock rate command is missing on the DCE serial interface.Correct answer▾
Why this is correct
The show controllers output clearly states “DCE V.35, no clock rate” and “no clock rate configured,” confirming that the DCE side lacks the required clock rate. Without it, the serial interface cannot bring the line protocol up.
✗The serial cable type is incorrectly identified.Wrong answer — click to see why▾
Why this is wrong here
Some candidates may misinterpret the DCE cable type as a problem, but the output merely states the detected cable type accurately, not a misidentification.
✗The encapsulation mismatch is causing the line protocol to stay down.Wrong answer — click to see why▾
Why this is wrong here
Candidates often fixate on encapsulation issues when line protocol is down, overlooking the explicit hardware-level clocking problem shown here.
✗The serial interface is administratively shut down.Wrong answer — click to see why▾
Why this is wrong here
Many candidates think that a non‑functioning interface might be shut down, but the exhibit explicitly shows an active hardware detection with a configuration error, not an administrative shutdown.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Go deeper
Related to this question
Learn chapter
ARP — Address Resolution Protocol
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
Key term
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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