- A
The server's TCP stack is corrupt.
Why wrong: TCP is not involved in ICMP communication; ICMP operates directly over IP without a transport layer protocol like TCP or UDP. A corrupt TCP stack would affect TCP-based applications, not ICMP echo replies.
- B
The server's ARP cache contains an incorrect MAC address for the destination host.
The server must resolve the destination IP address (Host A) to a MAC address to build the frame. If the ARP cache holds a wrong MAC, the frame carrying the echo reply is sent to an incorrect device or the encapsulation fails, so the reply never appears on the wire.
- C
The server's routing table is missing a route to the source network.
Why wrong: If the server had no route to Host A's network, it would not be able to send the reply at all, but the problem states the reply is generated. Also, the request's arrival implies a return route exists via the same path. The issue is at Layer 2, not Layer 3.
- D
The switch port connecting Server B has port security enabled and has learned a different MAC.
Why wrong: Port security would drop frames from disallowed source MACs. Since the initial request from Host A arrived, its source MAC is permitted. The server's reply uses its own source MAC, which is inherently allowed on its own port. Thus, port security would not block the reply.
Quick Answer
The answer is an incorrect MAC address in the server’s ARP cache. When Server B receives the ICMP echo request and generates a reply, the reply must be encapsulated in a Layer 2 frame for transmission. If the ARP cache holds a wrong MAC address for the destination host or the default gateway, the server will place that incorrect MAC in the frame’s destination field. The switch then either drops the frame or forwards it to the wrong device, so the reply never appears on the wire, even though the server successfully created it. On the CCNA 200-301 v2 exam, this scenario tests your understanding of how ARP resolution directly impacts Layer 3 reachability—a common trap is to blame routing or ACLs when the issue is actually a corrupted ARP entry. Remember: if packets arrive and replies are generated but vanish, think “ARP mismatch.” A helpful mnemonic is “Replies need right MAC—check the ARP stack.”
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.
A network administrator captures traffic on Server B and finds that ICMP echo requests from Host A arrive, and the server generates corresponding echo replies, but these replies never appear on the wire. The server's routing table has a valid default gateway, and no ACLs are blocking the traffic. What is the most likely cause?
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.
Clue:
"never"Why it matters: Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.
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 server's ARP cache contains an incorrect MAC address for the destination host.
The ICMP echo requests arrive at Server B, and the server generates replies, but they never appear on the wire. This indicates the server cannot deliver the frames to the next hop. Since the server has a valid default gateway and no ACLs are blocking, the most likely cause is an incorrect MAC address in the ARP cache for the destination (either the host or the default gateway). The server will encapsulate the IP packet into a frame using the wrong MAC, causing the switch to drop the frame or send it to the wrong device, so the reply never reaches the wire correctly.
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 server's TCP stack is corrupt.
- ✓
The server's ARP cache contains an incorrect MAC address for the destination host.
Why this is correct
The server must resolve the destination IP address (Host A) to a MAC address to build the frame. If the ARP cache holds a wrong MAC, the frame carrying the echo reply is sent to an incorrect device or the encapsulation fails, so the reply never appears on the wire.
Clue confirmation
The clue words "most likely", "never" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The server's routing table is missing a route to the source network.
Why it's wrong here
If the server had no route to Host A's network, it would not be able to send the reply at all, but the problem states the reply is generated. Also, the request's arrival implies a return route exists via the same path. The issue is at Layer 2, not Layer 3.
- ✗
The switch port connecting Server B has port security enabled and has learned a different MAC.
Why it's wrong here
Port security would drop frames from disallowed source MACs. Since the initial request from Host A arrived, its source MAC is permitted. The server's reply uses its own source MAC, which is inherently allowed on its own port. Thus, port security would not block the reply.
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 server's ARP cache contains an incorrect MAC address for the destination host.Correct answer▾
Why this is correct
The server must resolve the destination IP address (Host A) to a MAC address to build the frame. If the ARP cache holds a wrong MAC, the frame carrying the echo reply is sent to an incorrect device or the encapsulation fails, so the reply never appears on the wire.
✗The server's TCP stack is corrupt.Wrong answer — click to see why▾
Why this is wrong here
ICMP does not use TCP; it is encapsulated directly in IP. The symptom would be unrelated to TCP.
✗The server's routing table is missing a route to the source network.Wrong answer — click to see why▾
Why this is wrong here
A missing route would prevent the IP layer from even attempting to send the packet, not result in a generated-but-not-transmitted error.
✗The switch port connecting Server B has port security enabled and has learned a different MAC.Wrong answer — click to see why▾
Why this is wrong here
Port security would have blocked the incoming request if Host A's MAC violated the policy, but the request was received, so this cannot explain the missing reply.
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 Layer 3 routing (which works correctly here) and Layer 2 frame delivery (which fails due to ARP issues), leading candidates to incorrectly blame routing or ACLs instead of the ARP cache.
Detailed technical explanation
How to think about this question
When a host sends an IP packet to a destination on a different subnet, it must resolve the next-hop IP (usually the default gateway) to a MAC address using ARP. If the ARP cache has a stale or incorrect entry (e.g., due to a duplicate IP or a previous network change), the frame is sent to the wrong MAC, and the reply never reaches the intended device. This can be verified with 'show ip arp' or 'arp -a' on the server. In real-world scenarios, this often occurs after a router replacement or when a static ARP entry is misconfigured.
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
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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 server's ARP cache contains an incorrect MAC address for the destination host. — The ICMP echo requests arrive at Server B, and the server generates replies, but they never appear on the wire. This indicates the server cannot deliver the frames to the next hop. Since the server has a valid default gateway and no ACLs are blocking, the most likely cause is an incorrect MAC address in the ARP cache for the destination (either the host or the default gateway). The server will encapsulate the IP packet into a frame using the wrong MAC, causing the switch to drop the frame or send it to the wrong device, so the reply never reaches the wire correctly.
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", "never". 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
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Last reviewed: Jun 25, 2026
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