- A
The IP addresses on the serial interfaces are in different subnets, so packets are dropped at Layer 3.
Why wrong: A Layer 3 subnet mismatch would cause pings to fail, but the line protocol would still come up if Layer 2 is functioning correctly.
- B
The mismatched encapsulation types prevent the routers from forming a Layer 2 connection, so the line protocol remains down.
With PPP on one end and HDLC on the other, the L2 frames are incompatible. The receiving router cannot decode the incoming frame, and L2 keepalives fail, causing the line protocol to drop.
- C
The serial cable is faulty, causing physical layer issues that trigger the line protocol down state.
Why wrong: If the physical layer were faulty, the interface would show 'Serial0/0 is down', not 'up'.
- D
The routers are missing a routing protocol configuration, so they cannot route traffic across the link.
Why wrong: A direct serial link between two routers in the same subnet does not require a routing protocol for ping to work; the connected route would be used.
Quick Answer
The answer is a serial encapsulation mismatch, which causes the line protocol to remain down even when the physical layer is up. This occurs because the Layer 2 encapsulation type—such as PPP or HDLC—must match on both ends of a serial link for the routers to establish a valid connection. On the CCNA 200-301 v2 exam, this scenario tests your understanding of serial interface troubleshooting, often appearing as a show interfaces output where Serial0/0 is up but line protocol is down. A common trap is assuming the physical layer issue is at fault, but the real culprit is the mismatch between R1’s encapsulation ppp and R2’s default hdlc. Remember, HDLC is Cisco’s default serial encapsulation, so if one side is changed to PPP without updating the other, the line protocol will drop. A helpful memory tip: “PPP and HDLC don’t shake hands—match the wrap to keep the link from collapsing.”
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.
Two routers, R1 and R2, are connected via a serial link. The interface on R1 shows 'Serial0/0 is up, line protocol is down' and no pings succeed across the link. You check the configuration and notice R1 has 'encapsulation ppp' but R2's serial interface was mistakenly left at the default encapsulation hdlc.
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 mismatched encapsulation types prevent the routers from forming a Layer 2 connection, so the line protocol remains down.
Option B is correct because the line protocol on a serial interface requires both ends to agree on the Layer 2 encapsulation type. R1 is configured with PPP (encapsulation ppp), while R2 defaults to HDLC. Since these encapsulations are incompatible, the routers cannot establish a valid Layer 2 connection, causing the line protocol to remain down despite the physical layer being up.
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 IP addresses on the serial interfaces are in different subnets, so packets are dropped at Layer 3.
Why it's wrong here
A Layer 3 subnet mismatch would cause pings to fail, but the line protocol would still come up if Layer 2 is functioning correctly.
- ✓
The mismatched encapsulation types prevent the routers from forming a Layer 2 connection, so the line protocol remains down.
Why this is correct
With PPP on one end and HDLC on the other, the L2 frames are incompatible. The receiving router cannot decode the incoming frame, and L2 keepalives fail, causing the line protocol to drop.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The serial cable is faulty, causing physical layer issues that trigger the line protocol down state.
Why it's wrong here
If the physical layer were faulty, the interface would show 'Serial0/0 is down', not 'up'.
- ✗
The routers are missing a routing protocol configuration, so they cannot route traffic across the link.
Why it's wrong here
A direct serial link between two routers in the same subnet does not require a routing protocol for ping to work; the connected route would be used.
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 mismatched encapsulation types prevent the routers from forming a Layer 2 connection, so the line protocol remains down.Correct answer▾
Why this is correct
With PPP on one end and HDLC on the other, the L2 frames are incompatible. The receiving router cannot decode the incoming frame, and L2 keepalives fail, causing the line protocol to drop.
✗The IP addresses on the serial interfaces are in different subnets, so packets are dropped at Layer 3.Wrong answer — click to see why▾
Why this is wrong here
The line protocol down state is a Layer 2 indicator, not a Layer 3 problem. It reflects the failure of the data link layer to establish a connection, independent of IP addressing.
✗The serial cable is faulty, causing physical layer issues that trigger the line protocol down state.Wrong answer — click to see why▾
Why this is wrong here
The 'interface up' part of the status explicitly confirms that the physical layer (Layer 1) is operational. A faulty cable would cause both physical and line protocol to show down.
✗The routers are missing a routing protocol configuration, so they cannot route traffic across the link.Wrong answer — click to see why▾
Why this is wrong here
Line protocol down indicates a Layer 2 failure, which occurs before any routing decision. Even without a routing protocol, the line protocol would come up if Layer 2 were functional, because the interface status is independent of routing.
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 'line protocol is down' (Layer 2 issue) and 'Serial0/0 is down' (Layer 1 issue), and candidates mistakenly attribute a line protocol down state to physical problems or IP addressing errors rather than encapsulation mismatch.
Trap categories for this question
Command / output trap
If the physical layer were faulty, the interface would show 'Serial0/0 is down', not 'up'.
Detailed technical explanation
How to think about this question
PPP and HDLC are both Layer 2 encapsulation protocols used on serial links, but they use different frame formats and control mechanisms. Cisco HDLC includes a proprietary Type field, while PPP uses a standardized protocol field and supports authentication (PAP/CHAP) and multilink. When encapsulations mismatch, the routers cannot exchange keepalive frames or LCP packets, so the line protocol stays down. In real-world troubleshooting, the 'show interfaces serial' command output is critical: a 'line protocol is down' with 'up/down' status almost always points to a Layer 2 issue like encapsulation mismatch, clocking problems, or keepalive failure.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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: The mismatched encapsulation types prevent the routers from forming a Layer 2 connection, so the line protocol remains down. — Option B is correct because the line protocol on a serial interface requires both ends to agree on the Layer 2 encapsulation type. R1 is configured with PPP (encapsulation ppp), while R2 defaults to HDLC. Since these encapsulations are incompatible, the routers cannot establish a valid Layer 2 connection, causing the line protocol to remain down despite the physical layer being up.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.
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