Question 1,864 of 2,152

DHCP (IPv4 and IPv6) →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is a DHCPv4 client identifier format mismatch, specifically that the Cisco router sends the client identifier as a concatenation of hardware type (01) and MAC address (e.g., 0100.1122.3344), while the ISP modem expects only the raw MAC address. This non-standard format causes the modem to ignore the DISCOVER messages, resulting in no OFFER despite the interface being up/up and other devices working fine. On the CCNP ENARSI 300-410 exam, this scenario tests your understanding of DHCP client behavior and the `ip dhcp client client-id` command, which forces the router to send just the MAC address. A common trap is assuming a Layer 1 or 2 issue when the real problem is a DHCP option mismatch at Layer 3. Remember the mnemonic: “Cisco adds a 01, ISP wants it gone” — if you see DISCOVER but no OFFER, check the client-id format first.

300-410 DHCP (IPv4 and IPv6) Practice Question

This 300-410 practice question tests your understanding of dhcp (ipv4 and ipv6). 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.

An engineer is troubleshooting a DHCPv4 issue where a Cisco router acting as a DHCP client on interface Gi0/0 is not receiving an IP address from an ISP modem. The router has 'ip address dhcp' on the interface. The engineer sees that the interface is up/up, but no IP address is assigned. Debug shows that the router is sending DHCP DISCOVER messages but receives no OFFER. The ISP modem is known to work with other devices. 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.

Question 1mediummultiple choice

Read the full DHCP explanation →

A
The router needs the 'ip dhcp client broadcast-flag' command.
Why wrong: Incorrect because the broadcast flag is used to request a broadcast reply; it does not affect the DISCOVER being ignored.
B
The router is sending the client identifier as the MAC address in a non-standard format; the modem expects the client identifier to be the MAC address only.
Correct because Cisco routers by default send the client identifier as the MAC address with a type byte (0x01), while some modems expect only the MAC address; configuring 'ip dhcp client client-id' with the correct format resolves the issue.
C
The router's interface is in a VRF, and the DHCP client needs VRF awareness.
Why wrong: Incorrect because the interface is not mentioned to be in a VRF; this is a standard DHCP client scenario.
D
The ISP modem requires DHCP Option 82 to be present.
Why wrong: Incorrect because Option 82 is typically added by relay agents, not clients; modems rarely require it.

Full breakdown with real-world context →

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 router is sending the client identifier as the MAC address in a non-standard format; the modem expects the client identifier to be the MAC address only.

The router is sending DHCP DISCOVER messages but receiving no OFFER, indicating the ISP modem is ignoring the requests. By default, Cisco routers send the client identifier in DHCP packets as a concatenation of the hardware type (1 for Ethernet) and the MAC address (e.g., '0100.1122.3344'). Many ISP modems expect the client identifier to be just the MAC address (without the hardware type prefix), or they may not use client identifier matching at all. The 'ip dhcp client client-id' command can be used to override this behavior, making the router send only the MAC address as the client identifier, which resolves the incompatibility.

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 router needs the 'ip dhcp client broadcast-flag' command.
Why it's wrong here
Incorrect because the broadcast flag is used to request a broadcast reply; it does not affect the DISCOVER being ignored.
✓
The router is sending the client identifier as the MAC address in a non-standard format; the modem expects the client identifier to be the MAC address only.
Why this is correct
Correct because Cisco routers by default send the client identifier as the MAC address with a type byte (0x01), while some modems expect only the MAC address; configuring 'ip dhcp client client-id' with the correct format resolves the issue.
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 router's interface is in a VRF, and the DHCP client needs VRF awareness.
Why it's wrong here
Incorrect because the interface is not mentioned to be in a VRF; this is a standard DHCP client scenario.
✗
The ISP modem requires DHCP Option 82 to be present.
Why it's wrong here
Incorrect because Option 82 is typically added by relay agents, not clients; modems rarely require it.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the subtle default behavior of the DHCP client identifier format, where candidates assume the MAC address alone is sent, but the router prepends a hardware type byte, causing incompatibility with non-Cisco servers.

Trap categories for this question

Scenario analysis trap
Incorrect because the interface is not mentioned to be in a VRF; this is a standard DHCP client scenario.

Detailed technical explanation

How to think about this question

The DHCP client identifier (Option 61) is used by servers to uniquely identify clients and maintain lease state. Cisco routers, by default, construct the client identifier as a 1-byte hardware type (0x01 for Ethernet) followed by the MAC address (e.g., 01:00:11:22:33:44). Some ISP modems or third-party DHCP servers expect the client identifier to be only the MAC address (without the type byte) or may not support Option 61 at all. This mismatch causes the server to silently drop the DISCOVER because it cannot match the client identifier to an expected format. The fix is to configure 'ip dhcp client client-id ascii <string>' or 'ip dhcp client client-id hex <value>' to send a compatible identifier.

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.

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FAQ

Questions learners often ask

What does this 300-410 question test?

DHCP (IPv4 and IPv6) — This question tests DHCP (IPv4 and IPv6) — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The router is sending the client identifier as the MAC address in a non-standard format; the modem expects the client identifier to be the MAC address only. — The router is sending DHCP DISCOVER messages but receiving no OFFER, indicating the ISP modem is ignoring the requests. By default, Cisco routers send the client identifier in DHCP packets as a concatenation of the hardware type (1 for Ethernet) and the MAC address (e.g., '0100.1122.3344'). Many ISP modems expect the client identifier to be just the MAC address (without the hardware type prefix), or they may not use client identifier matching at all. The 'ip dhcp client client-id' command can be used to override this behavior, making the router send only the MAC address as the client identifier, which resolves the incompatibility.

What should I do if I get this 300-410 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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