The answer is that the ACL permits only TCP and UDP but not ESP packets, which blocks IPsec ESP protocol traffic. This is correct because IPsec ESP operates as IP protocol 50, a distinct layer 4 protocol separate from TCP (protocol 6) or UDP (protocol 17), so an ACL that explicitly permits only TCP and UDP will drop ESP packets via the implicit deny rule. While IKE negotiation using UDP ports 500 and 4500 may succeed, the actual encrypted data traffic fails because ESP packets are never allowed through the firewall. On the CISSP exam, this tests your understanding of IPsec architecture and the critical distinction between IKE (a UDP-based control protocol) and ESP (a separate IP protocol) — a common trap is assuming that permitting IKE alone is sufficient for IPsec. Remember the memory tip: ESP is not TCP or UDP; it’s protocol 50, so your ACL must explicitly permit IP protocol 50, not just application ports.
CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and network security. 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
access-list 101 permit tcp any host 192.168.1.10 eq 443
access-list 101 permit udp any host 192.168.1.10 eq 500
access-list 101 permit udp any host 192.168.1.10 eq 4500
access-list 101 deny ip any any
Refer to the exhibit. A network administrator sees that IPsec IKE negotiations fail between site A and site B. Site B's firewall has the above ACL applied inbound on the external interface. 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 ACL permits only TCP and UDP but not ESP packets.
The ACL permits TCP and UDP only. IPsec ESP uses IP protocol 50 (not TCP/UDP), so ESP packets are denied by the implicit deny rule. While IKE (UDP 500/4500) is permitted, the actual data traffic after IKE negotiation fails because ESP is blocked.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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 ACL permits only TCP and UDP but not ESP packets.
The ACL permits only TCP and UDP but not AH packets.
Why it's wrong here
AH uses IP protocol 51, but AH is less common; the issue is more likely ESP.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A security analyst at a medium-sized enterprise encounters this scenario during an investigation or architecture review. The correct answer reflects best practice for the specific threat or control described. ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement. Security exam questions test whether you can match controls to threats in context — not just recall definitions.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CISSP ACL questions on filtering logic and placement.
Communication and Network Security — This question tests Communication and Network Security — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The ACL permits only TCP and UDP but not ESP packets. — The ACL permits TCP and UDP only. IPsec ESP uses IP protocol 50 (not TCP/UDP), so ESP packets are denied by the implicit deny rule. While IKE (UDP 500/4500) is permitted, the actual data traffic after IKE negotiation fails because ESP is blocked.
What should I do if I get this CISSP question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CISSP ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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