Question 420 of 2,011
hardMultiple ChoiceObjective-mapped
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
Incorrect because 'ipv6 address ... eui-64' enables IPv6 on the interface; 'ipv6 enable' is not required.
- ✓
The ISP's DHCPv6 server is not configured to delegate a prefix.
Why this is correct
Correct because if the server does not delegate a prefix, the client's 'ipv6 dhcp client pd' request will fail, and the LAN interfaces will not receive any addresses from the delegated prefix.
- ✗
The 'ipv6 dhcp client pd' command should be 'ipv6 dhcp client pd ISP-PREFIX hint ::/48'.
Why it's wrong here
Incorrect because the 'hint' option is optional and does not prevent delegation; the server may still delegate a different prefix.
- ✗
The WAN interface needs the 'ipv6 nd other-config-flag' command.
Why it's wrong here
Incorrect because the 'other-config-flag' is for LAN interfaces to indicate additional information; it is not related to prefix delegation on the WAN.
Visual reference
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Last reviewed: Jun 24, 2026
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