CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
A
The clock rate command is missing on the DCE serial interface.
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.
B
The serial cable type is incorrectly identified.
Why wrong: The output correctly identifies the cable as V.35 DCE. No incorrect identification is present; the cable type shown matches the physical connection.
C
The encapsulation mismatch is causing the line protocol to stay down.
Why wrong: 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.
D
The serial interface is administratively shut down.
Why wrong: 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.
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The serial cable type is incorrectly identified.
Why it's wrong here
The output correctly identifies the cable as V.35 DCE. No incorrect identification is present; the cable type shown matches the physical connection.
✗
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?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Trap categories for this question
Command / output trap
The output correctly identifies the cable as V.35 DCE. No incorrect identification is present; the cable type shown matches the physical connection.
Detailed technical explanation
How to think about this question
On Cisco routers, the DCE end of a serial cable must supply a clock signal using the 'clock rate' command (e.g., 'clock rate 64000' for 64 kbps). Without this, the DTE end receives no timing signal, causing the physical layer to fail. In real-world scenarios, misconfigured clock rates can also cause CRC errors or intermittent connectivity if the rate exceeds the cable's capacity (e.g., > 2 Mbps on a V.35 cable).
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 200-301 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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