The answer is that the packet is being encapsulated after routing, such as by IPsec, which adds overhead and causes the packet to exceed the interface MTU of 1500 bytes. This is correct because the firewall makes its routing decision based on the original 1500-byte packet, but subsequent encapsulation—typically adding 50–60 bytes for IPsec ESP headers and trailers—inflates the total frame size beyond the configured MTU. The drop occurs at the physical interface after encapsulation, which is why the log shows 'packet too long for interface MTU' even though the original packet fits. On the PCNSE exam, this scenario tests your understanding of the order of operations: routing happens before encapsulation, so the MTU check fails post-encapsulation. A common trap is assuming the MTU check occurs before encapsulation, or that a 1500-byte packet will always pass a 1500 MTU interface. Remember the memory tip: "Route first, wrap second—if it’s too big, it gets rejected."
PCNSE Core Concepts and Architecture Practice Question
This PCNSE practice question tests your understanding of core concepts and architecture. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 firewall administrator notices that traffic from a specific subnet is being unexpectedly dropped. The firewall log shows a 'flow_drop' reason of 'packet too long for interface MTU'. The interface MTU is set to 1500, and the packets are 1500 bytes. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The packet is being encapsulated (e.g., IPsec) after routing, increasing its size beyond 1500 bytes.
When a packet is encapsulated (e.g., by IPsec) after the routing decision, the original packet's size remains 1500 bytes, but the encapsulation adds overhead (e.g., IPsec ESP headers/trailers, typically 50–60 bytes). This causes the resulting frame to exceed the interface MTU of 1500, triggering a 'packet too long for interface MTU' drop. The firewall logs the drop at the physical interface after encapsulation, not before.
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 route lookup for the destination requires a larger MTU.
Why it's wrong here
Route lookup does not alter packet size; the forwarding interface MTU is checked.
✗
The firewall is not performing TCP MSS clamping on the traffic.
Why it's wrong here
MSS clamping reduces segment size; the issue is after firewall processing, the packet becomes too large.
✗
The firewall is using jumbo frames on the internal interface.
Why it's wrong here
Jumbo frames allow larger packets, not cause drops due to MTU.
✓
The packet is being encapsulated (e.g., IPsec) after routing, increasing its size beyond 1500 bytes.
Why this is correct
Encapsulation adds headers; if the original packet is near MTU, the encapsulated packet exceeds it.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume the firewall drops the packet before encapsulation because the original packet matches the MTU, but the drop occurs after encapsulation adds overhead, making the final frame too large.
Detailed technical explanation
How to think about this question
In Palo Alto Networks firewalls, IPsec tunnel encapsulation occurs after the route lookup and before the packet is transmitted on the physical interface. The firewall's MTU check is performed on the final frame size, including all tunnel headers. A common workaround is to reduce the interface MTU or enable TCP MSS clamping (set to 1350–1400) to account for IPsec overhead, but the root cause is the post-routing encapsulation increasing the packet size beyond the egress MTU.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Core Concepts and Architecture — This question tests Core Concepts and Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The packet is being encapsulated (e.g., IPsec) after routing, increasing its size beyond 1500 bytes. — When a packet is encapsulated (e.g., by IPsec) after the routing decision, the original packet's size remains 1500 bytes, but the encapsulation adds overhead (e.g., IPsec ESP headers/trailers, typically 50–60 bytes). This causes the resulting frame to exceed the interface MTU of 1500, triggering a 'packet too long for interface MTU' drop. The firewall logs the drop at the physical interface after encapsulation, not before.
What should I do if I get this PCNSE 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.
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Question Discussion
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