- A
Configure the clock rate on R1's serial interface.
On a serial DCE cable, the DCE end must provide clocking. Without a clock rate, the interface is down/down. Setting 'clock rate 128000' on R1 provides the necessary timing for the link to come up.
- B
Configure the clock rate on R2's serial interface.
Why wrong: This is incorrect because R2 is the DTE end of the serial link. The DTE does not configure clock rate; it receives clocking from the DCE. Configuring clock rate on R2 would be ignored or cause an error.
- C
Replace the serial cable with a straight-through cable.
Why wrong: This is incorrect because serial links use DCE/DTE crossover cables, not straight-through cables. The cable type is not the issue; the missing clock rate is.
- D
Set the encapsulation on both ends to PPP.
Why wrong: This is incorrect because the default encapsulation (HDLC) is fine and does not cause a down/down state. Changing encapsulation to PPP would not resolve the missing clock rate issue.
Serial Link DCE Clock Rate — Why It's Needed
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.
You are connected to the console of R1. The network has a point-to-point serial link between R1 and R2. The link is down and the line protocol is down. The cable is a DCE/DTE crossover, and R1 is the DCE. The initial configuration shows the interface with an IP address but no clock rate set.
Quick Answer
The correct answer is to configure the clock rate on R1’s serial interface because on a point-to-point serial link, the DCE (Data Circuit-terminating Equipment) end is responsible for providing the clock signal that synchronizes data transmission. Without a clock rate set on the DCE side, the physical layer cannot generate the necessary timing pulses, causing both the line protocol and the interface to remain in a down state. On the CCNA 200-301 v2 exam, this scenario tests your understanding of serial link DCE clock rate requirements, often appearing as a troubleshooting question where a link is down despite correct IP addressing. A common trap is assuming both ends need a clock rate, but only the DCE side configures it; the DTE side automatically receives timing. Remember the mnemonic “DCE Delivers Clock” to recall that the DCE end must always provide the clock rate for the serial link to come up.
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
Configure the clock rate on R1's serial interface.
On a serial DCE cable, the DCE end must configure a clock rate to provide timing. Without it, the line protocol stays down. Setting 'clock rate 128000' on R1 resolved the issue.
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.
- ✓
Configure the clock rate on R1's serial interface.
Why this is correct
On a serial DCE cable, the DCE end must provide clocking. Without a clock rate, the interface is down/down. Setting 'clock rate 128000' on R1 provides the necessary timing for the link to come up.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Configure the clock rate on R2's serial interface.
Why it's wrong here
This is incorrect because R2 is the DTE end of the serial link. The DTE does not configure clock rate; it receives clocking from the DCE. Configuring clock rate on R2 would be ignored or cause an error.
- ✗
Replace the serial cable with a straight-through cable.
Why it's wrong here
This is incorrect because serial links use DCE/DTE crossover cables, not straight-through cables. The cable type is not the issue; the missing clock rate is.
- ✗
Set the encapsulation on both ends to PPP.
Why it's wrong here
This is incorrect because the default encapsulation (HDLC) is fine and does not cause a down/down state. Changing encapsulation to PPP would not resolve the missing clock rate issue.
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.
✓Configure the clock rate on R1's serial interface.Correct answer▾
Why this is correct
On a serial DCE cable, the DCE end must provide clocking. Without a clock rate, the interface is down/down. Setting 'clock rate 128000' on R1 provides the necessary timing for the link to come up.
✗Configure the clock rate on R2's serial interface.Wrong answer — click to see why▾
Why this is wrong here
The DTE end does not set clock rate; only the DCE end does.
Why candidates choose this
Candidates may think both ends need clock rate or confuse DCE/DTE roles.
✗Replace the serial cable with a straight-through cable.Wrong answer — click to see why▾
Why this is wrong here
Serial cables are always DCE/DTE crossover; straight-through cables are for Ethernet.
Why candidates choose this
Candidates might think a cable mismatch causes the down/down state, but the cable is correct.
✗Set the encapsulation on both ends to PPP.Wrong answer — click to see why▾
Why this is wrong here
Encapsulation mismatch causes protocol down, not line protocol down; the line protocol is down due to no clock.
Why candidates choose this
Candidates may recall that encapsulation mismatch can cause issues, but here the problem is at Layer 1.
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.
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 200-301 question test?
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: Configure the clock rate on R1's serial interface. — On a serial DCE cable, the DCE end must configure a clock rate to provide timing. Without it, the line protocol stays down. Setting 'clock rate 128000' on R1 resolved the issue.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jun 7, 2026
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