hardMultiple Choice
300-410 Practice Question: An engineer is troubleshooting a DHCPv6 prefix…
An engineer is troubleshooting a DHCPv6 prefix delegation scenario. The router (R1) is a DHCPv6 client on its WAN interface and is supposed to receive a /48 prefix from the ISP's DHCPv6 server to delegate to LAN interfaces. The WAN interface is configured with 'ipv6 address dhcp' and 'ipv6 dhcp client pd ISP-PREFIX'. The LAN interfaces have 'ipv6 address ISP-PREFIX 0:0:0:1::/64 eui-64'. The engineer sees that the WAN interface gets a global IPv6 address, but the LAN interfaces do not get any IPv6 address. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between DHCPv6 address assignment (IA_NA) and prefix delegation (IA_PD), trapping candidates who assume that obtaining a WAN address automatically implies a delegated prefix is also received.
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 ISP's DHCPv6 server is not configured to delegate a prefix.
The WAN interface successfully obtains a global IPv6 address via DHCPv6, but the LAN interfaces lack IPv6 addresses because the DHCPv6 client process on R1 has not received a delegated prefix. The 'ipv6 dhcp client pd ISP-PREFIX' command requests a prefix delegation from the ISP's DHCPv6 server; if the server is not configured to delegate prefixes (e.g., missing the 'prefix-delegation' pool or 'ipv6 dhcp server' with prefix assignment), the client never receives a prefix to assign to LAN interfaces. This is the most likely cause given the symptom.
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 LAN interfaces are missing the 'ipv6 enable' command.
Why it's wrong here
The eui-64 address already enables IPv6 on the LAN interfaces, and ipv6 enable only generates a link-local address; it cannot create the missing global address. It is tempting because ipv6 enable is a common prerequisite, but here the interface already runs IPv6, so the delegated prefix is the actual dependency.
- ✓
The ISP's DHCPv6 server is not configured to delegate a prefix.
Why this is correct
Prefix delegation requires the ISP's DHCPv6 server to include IA_PD in its reply. The WAN interface obtaining a global address proves basic DHCPv6 works, but LAN interfaces remain unaddressed because no delegated prefix was returned.
- ✗
The 'ipv6 dhcp client pd' command should be 'ipv6 dhcp client pd ISP-PREFIX hint ::/48'.
Why it's wrong here
The hint keyword only suggests a preferred prefix length to the server; it is optional and does not stop the delegated prefix binding to LAN interfaces. It is tempting because hints influence server allocation, but they would matter when requesting a specific size, not when the delegated prefix fails to apply.
- ✗
The WAN interface needs the 'ipv6 nd other-config-flag' command.
Why it's wrong here
The other-config-flag is an RA field telling hosts to fetch other settings via DHCPv6; it does not affect whether the router installs a delegated prefix on its LAN interfaces. It is tempting because it relates to DHCPv6 on the WAN, but it governs host autoconfiguration, not prefix delegation binding.
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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.