The answer is that the DHCP client is using a different MAC address in the DHCP packet than the source MAC in the Ethernet frame. This occurs because DHCP snooping, a security feature on the switch, performs a validation by comparing the source MAC address in the Layer 2 Ethernet frame against the chaddr (client hardware address) field inside the DHCP packet’s payload. When these two values do not match, the switch logs a “MAC Address Mismatch” and drops the packet to prevent a rogue client from spoofing another device’s identity to obtain a lease. On the ENCOR 350-401 exam, this concept tests your understanding of DHCP snooping’s role in mitigating DHCP starvation and spoofing attacks; a common trap is assuming the mismatch refers to the server’s MAC or the destination MAC. Remember the memory tip: “Frame MAC vs. chaddr—if they don’t match, the packet gets the snatch.”
350-401 Security Practice Question
This 350-401 practice question tests your understanding of security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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
Refer to the exhibit.
Switch# show ip dhcp snooping binding
MacAddress IpAddress Lease(sec) Type VLAN Interface
------------------ --------------- ---------- ------------- ---- --------------------
00:1A:2B:3C:4D:5E 192.168.1.10 86400 dhcp-snooping 10 GigabitEthernet0/1
00:1A:2B:3C:4D:5F 192.168.1.11 86400 dhcp-snooping 10 GigabitEthernet0/2
Switch# show ip dhcp snooping statistics
Packets Processed by DHCP Snooping = 100
Packets Dropped Because = 5
MAC Address Mismatch = 3
Invalid Server Replies = 2
Refer to the exhibit. A network administrator notices that some DHCP packets are being dropped due to 'MAC Address Mismatch'. What is the most likely cause of this drop?
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 DHCP client is using a different MAC address in the DHCP packet than the source MAC in the Ethernet frame.
The DHCP snooping feature on a switch compares the source MAC address in the Ethernet frame with the chaddr (client hardware address) field inside the DHCP packet. When a DHCP client sends a packet with a different MAC in the frame than in the chaddr field, the switch considers it a 'MAC Address Mismatch' and drops the packet. This security mechanism prevents a rogue client from spoofing another device's MAC address to obtain a lease.
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 DHCP server is sending packets with an incorrect server identifier option.
Why it's wrong here
Server identifier mismatch would be an invalid server reply.
✓
The DHCP client is using a different MAC address in the DHCP packet than the source MAC in the Ethernet frame.
Why this is correct
MAC address mismatch occurs when the chaddr field does not match the source MAC.
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.
✗
The DHCP client is sending a request with an incorrect transaction ID.
Why it's wrong here
Transaction ID mismatch is not a DHCP snooping drop reason.
✗
The DHCP offer packet is arriving on an untrusted port.
Why it's wrong here
That would cause 'Invalid Server Replies' drop.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between Layer 2 MAC checks (frame vs. chaddr) and Layer 3 or application-layer checks (server identifier, transaction ID), leading candidates to confuse DHCP snooping drops with client-side validation failures.
Detailed technical explanation
How to think about this question
DHCP snooping maintains a binding table that maps client MAC addresses to IP addresses and ports. The 'MAC Address Mismatch' check is performed on DHCP client messages (DISCOVER, REQUEST, DECLINE, RELEASE) received on untrusted ports. If the source MAC in the Ethernet header does not match the chaddr field in the DHCP payload, the packet is dropped to prevent MAC spoofing attacks. This is a Layer 2 security feature defined in RFC 2131 and implemented in Cisco IOS using the 'ip dhcp snooping' command.
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 practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Security — This question tests Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The DHCP client is using a different MAC address in the DHCP packet than the source MAC in the Ethernet frame. — The DHCP snooping feature on a switch compares the source MAC address in the Ethernet frame with the chaddr (client hardware address) field inside the DHCP packet. When a DHCP client sends a packet with a different MAC in the frame than in the chaddr field, the switch considers it a 'MAC Address Mismatch' and drops the packet. This security mechanism prevents a rogue client from spoofing another device's MAC address to obtain a lease.
What should I do if I get this 350-401 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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