mediumMultiple Choice
300-410 Practice Question: An engineer is troubleshooting a DHCPv4 issue…
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?
⚠ Common exam trap
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.
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.
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
The `ip dhcp client broadcast-flag` command forces the DHCP client to set the broadcast flag in DHCP discover messages, requesting that the server reply via broadcast rather than unicast. However, this flag only controls the reply addressing; it does not change the client identifier or any other field that the ISP modem might reject. The router's DISCOVER being ignored is caused by the client identifier format mismatch, not by the absence of the broadcast flag, so enabling this command would not resolve the issue.
- ✓
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.
- ✗
The router's interface is in a VRF, and the DHCP client needs VRF awareness.
Why it's wrong here
VRF awareness for a DHCP client is only relevant if the interface is configured with an IP VRF forwarding statement that places it in a specific VRF context. In the standard DHCP client scenario described, no VRF configuration is mentioned; the router is likely using the global routing table. Even if a VRF were involved, modern IOS supports DHCP client operation within a VRF, and the symptom of a modem ignoring DISCOVER messages is more directly tied to the client identifier encoding, not to VRF routing or forwarding. Thus, this option incorrectly introduces a VRF-related cause that does not apply to the given situation.
- ✗
The ISP modem requires DHCP Option 82 to be present.
Why it's wrong here
DHCP Option 82 (Relay Agent Information) is inserted by relay agents when forwarding DHCP requests across a Layer 3 boundary, not by the DHCP client itself. An ISP modem acting as a DHCP server or gateway may expect certain options, but Option 82 is not typically required from a directly connected client and is not a common cause of a DISCOVER message being ignored. The likely failure is due to the modem's strict parsing of the client identifier field, which the Cisco router formats incorrectly by prepending a type byte, not due to a missing relay agent option. Hence, this is not the correct explanation.
Visual reference
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.