The correct answer is that the DCE end of the serial link is missing a clock rate configuration. When a serial interface shows “line protocol is down” while the physical layer is up, it indicates that Layer 2 keepalives are failing, typically because the receiving router cannot synchronize bit timing. On a serial link, the DCE device must provide a clock signal using the clock rate command; without it, the DTE end never receives proper timing, so the line protocol stays down. On the CCNA 200-301 v2 exam, this scenario tests your understanding of serial WAN troubleshooting and the distinction between DCE and DTE roles. A common trap is assuming a cable fault or misconfiguration of encapsulation, but the missing clock rate is the specific cause when the interface is up yet the protocol is down. Remember the memory tip: “DCE must clock, or the protocol will lock.”
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 interfaces Serial0/0/0
Serial0/0/0 is up, line protocol is down
Hardware is GT96K Serial
Internet address is 192.168.1.1/30
MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set
Keepalive set (10 sec)
DCE, no clock rate set
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Refer to the exhibit. A network engineer is troubleshooting a connectivity issue between two routers connected via a serial link. The engineer runs the show interfaces Serial0/0/0 command on R1. Based on the output, what is the most likely cause of the problem?
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 interfaces Serial0/0/0
Serial0/0/0 is up, line protocol is down
Hardware is GT96K Serial
Internet address is 192.168.1.1/30
MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set
Keepalive set (10 sec)
DCE, no clock rate set
Last input never, output never, output hang never
Last clearing of "show interface" counters never
A
The interface is configured with the wrong encapsulation type.
Why wrong: Encapsulation mismatch can cause line protocol down, but the output shows the default HDLC encapsulation with no indication of mismatch. Nothing in the exhibit suggests a mismatch, and the presence of the 'no clock rate set' message points to a different root cause.
B
The interface is administratively shut down.
Why wrong: If the interface were shut down, the status line would read 'Serial0/0/0 is administratively down, line protocol is down'. The actual output shows the interface is 'up', meaning it has been no shut and the physical layer is active.
C
The serial cable is disconnected or has a physical fault.
Why wrong: A disconnected or faulty cable results in a 'Serial0/0/0 is down, line protocol is down' status. Here, the interface is 'up', proving the cable is connected and the physical signal is detected. The issue is at Layer 2.
D
The DCE end of the serial link is missing a clock rate configuration.
The output explicitly states 'DCE, no clock rate set'. On a serial WAN link, the data communications equipment (DCE) must supply the clock signal. Without the clock rate command, the line protocol cannot come up, regardless of all other settings being correct.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The DCE end of the serial link is missing a clock rate configuration.
The output shows that the interface is up (line protocol is down), and the serial cable is physically connected. The absence of a clock rate on the DCE end of a serial link causes the line protocol to remain down because the receiving router cannot synchronize bit timing. Option D is correct because the DCE device must provide a clock signal for the serial link to establish Layer 2 connectivity.
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 interface is configured with the wrong encapsulation type.
Why it's wrong here
Encapsulation mismatch can cause line protocol down, but the output shows the default HDLC encapsulation with no indication of mismatch. Nothing in the exhibit suggests a mismatch, and the presence of the 'no clock rate set' message points to a different root cause.
✗
The interface is administratively shut down.
Why it's wrong here
If the interface were shut down, the status line would read 'Serial0/0/0 is administratively down, line protocol is down'. The actual output shows the interface is 'up', meaning it has been no shut and the physical layer is active.
✗
The serial cable is disconnected or has a physical fault.
Why it's wrong here
A disconnected or faulty cable results in a 'Serial0/0/0 is down, line protocol is down' status. Here, the interface is 'up', proving the cable is connected and the physical signal is detected. The issue is at Layer 2.
✓
The DCE end of the serial link is missing a clock rate configuration.
Why this is correct
The output explicitly states 'DCE, no clock rate set'. On a serial WAN link, the data communications equipment (DCE) must supply the clock signal. Without the clock rate command, the line protocol cannot come up, regardless of all other settings being correct.
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.
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 DCE end of the serial link is missing a clock rate configuration.Correct answer▾
Why this is correct
The output explicitly states 'DCE, no clock rate set'. On a serial WAN link, the data communications equipment (DCE) must supply the clock signal. Without the clock rate command, the line protocol cannot come up, regardless of all other settings being correct.
✗The interface is configured with the wrong encapsulation type.Wrong answer — click to see why▾
Why this is wrong here
Many candidates assume that a Layer 2 protocol down on a serial link is always caused by an encapsulation mismatch, overlooking the explicit clocking issue displayed in the output.
✗The interface is administratively shut down.Wrong answer — click to see why▾
Why this is wrong here
Novice engineers might misinterpret 'line protocol is down' as an indication that the interface is disabled, without reading the full status line.
✗The serial cable is disconnected or has a physical fault.Wrong answer — click to see why▾
Why this is wrong here
The trap: candidates see 'line protocol is down' and immediately think of a physical problem, missing the clear distinction that the interface itself is 'up'.
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 'interface is up, line protocol is down' (Layer 1 up, Layer 2 down) and 'interface is down, line protocol is down' (Layer 1 fault), tricking candidates into thinking a physical cable issue is the cause when the real problem is a missing clock rate on the DCE.
Trap categories for this question
Command / output trap
Encapsulation mismatch can cause line protocol down, but the output shows the default HDLC encapsulation with no indication of mismatch. Nothing in the exhibit suggests a mismatch, and the presence of the 'no clock rate set' message points to a different root cause.
Detailed technical explanation
How to think about this question
On a serial link, the DCE (Data Circuit-terminating Equipment) end must have a clock rate configured using the 'clock rate' command under the interface; without it, the DTE (Data Terminal Equipment) cannot synchronize bit transitions, causing the line protocol to stay down even though the physical layer is up. In a lab or production environment, the DCE is typically the side with the female connector, and the 'show controllers serial' command can confirm which end is DCE. This scenario is common when connecting routers back-to-back without a CSU/DSU, as the clock rate must be manually set.
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 DCE end of the serial link is missing a clock rate configuration. — The output shows that the interface is up (line protocol is down), and the serial cable is physically connected. The absence of a clock rate on the DCE end of a serial link causes the line protocol to remain down because the receiving router cannot synchronize bit timing. Option D is correct because the DCE device must provide a clock signal for the serial link to establish Layer 2 connectivity.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
hard
✓ A.The clock rate command is missing on the DCE serial interface.
B.The serial cable type is incorrectly identified.
C.The encapsulation mismatch is causing the line protocol to stay down.
D.The serial interface is administratively shut down.
Why A: 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.
Last reviewed: Jun 25, 2026
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