The answer is HMAC-SHA1, as indicated by the “esp-sha-hmac” entry in the IPsec transform set output. This is correct because in Cisco IOS, the “esp-sha-hmac” keyword specifically maps to HMAC-SHA1, which uses the SHA-1 hash algorithm wrapped in an HMAC structure to produce a 96-bit integrity check value. On the CISSP exam, this tests your ability to interpret real-world device configurations and map them to cryptographic concepts—a common trap is confusing “esp-sha-hmac” with SHA-256 or assuming it means plain SHA-1 without HMAC. Remember that “hmac” in the keyword explicitly indicates keyed-hash message authentication, not just hashing. For a quick memory tip: think “SHA-1 + HMAC = 96-bit tag” whenever you see “esp-sha-hmac” in a show output.
CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and network security. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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
Router# show crypto ipsec sa peer 203.0.113.10 interface outside
interface: outside
path mtu 1500, ipsec overhead 58, media mtu 1500
current outbound spi: 0x12345678(305419896)
inbound esp sas:
spi: 0x87654321(2271560481)
transform: esp-aes-256 esp-sha-hmac
inbound ah sas:
outbound esp sas:
spi: 0x12345678(305419896)
transform: esp-aes-256 esp-sha-hmac
outbound ah sas:
Refer to the exhibit. Based on the output, which integrity algorithm is configured for the IPsec tunnel?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
HMAC-SHA1
The output shows 'esp-sha-hmac' under the transform set, which indicates the integrity algorithm is HMAC-SHA1 (SHA-1 hashing with HMAC). In Cisco IOS, 'esp-sha-hmac' specifically refers to HMAC-SHA1 (96-bit hash), not SHA-256 or MD5. This is the correct interpretation because the command 'crypto ipsec transform-set' uses 'esp-sha-hmac' to denote SHA-1-based integrity.
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.
✗
HMAC-MD5
Why it's wrong here
The output shows 'sha', not 'md5'.
✗
HMAC-SHA256
Why it's wrong here
The output does not specify SHA256.
✓
HMAC-SHA1
Why this is correct
The 'sha' in esp-sha-hmac stands for SHA1.
Related concept
Read the scenario before looking for a memorised answer.
✗
AES-GCM
Why it's wrong here
AES-GCM is an encryption mode that includes integrity, but the output shows separate esp-aes and esp-sha.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the specific naming convention in 'show crypto ipsec transform-set' output, where 'esp-sha-hmac' is easily mistaken for SHA-256 or generic SHA, but it strictly refers to HMAC-SHA1 (96-bit) in Cisco IOS.
Trap categories for this question
Command / output trap
The output shows 'sha', not 'md5'.
Detailed technical explanation
How to think about this question
IPsec transform sets define the security protocols and algorithms for the tunnel; 'esp-sha-hmac' uses HMAC with SHA-1 to produce a 96-bit integrity check value (ICV) appended to each ESP packet. Under the hood, SHA-1 processes data in 512-bit blocks and outputs a 160-bit hash, which is then truncated to 96 bits for the ICV. In real-world scenarios, SHA-1 is now considered deprecated for collision resistance, but many legacy IPsec VPNs still use it; migration to SHA-256 (esp-sha256-hmac) is recommended for stronger security.
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 developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: HMAC-SHA1 — The output shows 'esp-sha-hmac' under the transform set, which indicates the integrity algorithm is HMAC-SHA1 (SHA-1 hashing with HMAC). In Cisco IOS, 'esp-sha-hmac' specifically refers to HMAC-SHA1 (96-bit hash), not SHA-256 or MD5. This is the correct interpretation because the command 'crypto ipsec transform-set' uses 'esp-sha-hmac' to denote SHA-1-based integrity.
What should I do if I get this CISSP 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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Question Discussion
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