The answer is a keepalive failure, which causes the line protocol to remain down even though the physical layer is up. On a serial link using HDLC encapsulation, keepalive packets are exchanged every 10 seconds by default to maintain the Layer 2 connection; if these packets are not received, the router considers the data link layer inactive and places the protocol in a down state. This scenario directly tests your understanding of the serial interface states for the CCNA 200-301 v2 exam, where the classic "up/down" status isolates the problem to Layer 2 after verifying encapsulation matching and clock rate. A common trap is to suspect a cable fault or encapsulation mismatch, but since those are confirmed correct, the missing keepalives are the culprit. Memory tip: think of keepalives as the "heartbeat" of HDLC—no heartbeat, no protocol life.
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 ip interface brief
Interface IP-Address OK? Method Status Protocol
GigabitEthernet0/0 192.168.1.1 YES manual up up
GigabitEthernet0/1 unassigned YES unset administratively down down
Serial0/0 10.0.0.1 YES manual up up
Serial0/1 172.16.1.1 YES manual up down
Serial0/2 unassigned YES unset administratively down down
Loopback0 1.1.1.1 YES manual up up
Refer to the exhibit. A network engineer is troubleshooting a connectivity issue on R1. The serial link to R2 on interface Serial0/1 is using HDLC encapsulation, and the physical cable has been verified as good. The engineer has confirmed that the encapsulation type matches on both routers and that the clock rate is correctly configured on the DCE end. Based on the output, what is the most likely cause of the line protocol down state on Serial0/1?
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 ip interface brief
Interface IP-Address OK? Method Status Protocol
GigabitEthernet0/0 192.168.1.1 YES manual up up
GigabitEthernet0/1 unassigned YES unset administratively down down
Serial0/0 10.0.0.1 YES manual up up
Serial0/1 172.16.1.1 YES manual up down
Serial0/2 unassigned YES unset administratively down down
Loopback0 1.1.1.1 YES manual up up
A
The encapsulation type is mismatched between R1 and R2.
Why wrong: The scenario states that the encapsulation type has been verified as matching (HDLC) on both ends, so an encapsulation mismatch cannot be the cause.
B
The clock rate has not been configured on the DCE end of the serial link.
Why wrong: The scenario confirms that the clock rate is correctly configured on the DCE end, ruling out a clocking issue.
C
The IP address configured on Serial0/1 conflicts with another interface.
Why wrong: An IP address conflict would affect Layer 3 reachability, not the line protocol. Line protocol down is a Layer 2 issue, so an IP address mismatch is irrelevant here.
D
The keepalive packets are not being received, causing the protocol to stay down.
With encapsulation and clocking confirmed, the up/down state on a serial link running HDLC points to a keepalive failure—the router is not receiving keepalive messages from the other end.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The keepalive packets are not being received, causing the protocol to stay down.
The exhibit shows Serial0/1 with status 'up' and protocol 'down' (up/down). This indicates Layer 1 is operational but the Layer 2 Data Link layer is not. Since the engineer already verified that encapsulation (HDLC) matches on both ends and the clock rate is correctly set, the only remaining common cause for a serial interface in up/down state is a keepalive failure. HDLC sends periodic keepalive messages across the link; if they are not received, the line protocol remains down.
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 encapsulation type is mismatched between R1 and R2.
Why it's wrong here
The scenario states that the encapsulation type has been verified as matching (HDLC) on both ends, so an encapsulation mismatch cannot be the cause.
✗
The clock rate has not been configured on the DCE end of the serial link.
Why it's wrong here
The scenario confirms that the clock rate is correctly configured on the DCE end, ruling out a clocking issue.
✗
The IP address configured on Serial0/1 conflicts with another interface.
Why it's wrong here
An IP address conflict would affect Layer 3 reachability, not the line protocol. Line protocol down is a Layer 2 issue, so an IP address mismatch is irrelevant here.
✓
The keepalive packets are not being received, causing the protocol to stay down.
Why this is correct
With encapsulation and clocking confirmed, the up/down state on a serial link running HDLC points to a keepalive failure—the router is not receiving keepalive messages from the other end.
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 keepalive packets are not being received, causing the protocol to stay down.Correct answer▾
Why this is correct
With encapsulation and clocking confirmed, the up/down state on a serial link running HDLC points to a keepalive failure—the router is not receiving keepalive messages from the other end.
✗The encapsulation type is mismatched between R1 and R2.Wrong answer — click to see why▾
Why this is wrong here
Candidates might fixate on the up/down status and assume encapsulation mismatch without considering the explicitly stated troubleshooting steps.
✗The clock rate has not been configured on the DCE end of the serial link.Wrong answer — click to see why▾
Why this is wrong here
Students often associate up/down with missing clock rate, but the question precludes this by confirming correct configuration.
✗The IP address configured on Serial0/1 conflicts with another interface.Wrong answer — click to see why▾
Why this is wrong here
Some learners confuse Layer 3 problems (IP issues) with the line protocol status that reflects Layer 2 health.
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
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Trap categories for this question
Scenario analysis trap
The scenario states that the encapsulation type has been verified as matching (HDLC) on both ends, so an encapsulation mismatch cannot be the cause.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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.
Identify which 200-301 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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 keepalive packets are not being received, causing the protocol to stay down. — The exhibit shows Serial0/1 with status 'up' and protocol 'down' (up/down). This indicates Layer 1 is operational but the Layer 2 Data Link layer is not. Since the engineer already verified that encapsulation (HDLC) matches on both ends and the clock rate is correctly set, the only remaining common cause for a serial interface in up/down state is a keepalive failure. HDLC sends periodic keepalive messages across the link; if they are not received, the line protocol remains down.
What should I do if I get this 200-301 question wrong?
Identify which 200-301 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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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