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300-410 Practice Question: An engineer is troubleshooting a DHCPv6 stateful…
An engineer is troubleshooting a DHCPv6 stateful (DHCPv6) deployment. The router is configured as a DHCPv6 server with a pool for prefix 2001:db8:2::/64. Clients on the LAN are configured to use DHCPv6, but they are not receiving IPv6 addresses. The router interface has 'ipv6 address 2001:db8:2::1/64' and 'ipv6 dhcp server DHCP_POOL'. The engineer sees that the clients are sending SOLICIT messages, but the router sends no REPLY. What is the issue?
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 DHCPv6 pool is missing the 'address prefix 2001:db8:2::/64' command.
For stateful DHCPv6, the router must also send Router Advertisements with the 'M' (Managed) flag set to indicate that clients should use DHCPv6. Without the 'ipv6 nd managed-config-flag' command, clients may not request addresses via DHCPv6, or the server may not respond appropriately. However, the symptom says clients are sending SOLICIT messages, so they are attempting DHCPv6. The router is not replying, which could be due to a misconfigured pool or the server not being enabled. The most likely cause is that the DHCPv6 pool does not have an 'address prefix' statement, so the server has no addresses to assign.
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 interface is missing the 'ipv6 nd managed-config-flag' command.
Why it's wrong here
Incorrect because clients are already sending SOLICIT messages, indicating they are using DHCPv6; the M flag is not required for the server to respond to SOLICITs.
- ✓
The DHCPv6 pool is missing the 'address prefix 2001:db8:2::/64' command.
Why this is correct
Correct because without an address prefix in the pool, the DHCPv6 server has no addresses to assign and will not send a REPLY to SOLICIT messages.
- ✗
The router needs the 'ipv6 dhcp relay' command on the interface.
Why it's wrong here
Incorrect because the router is acting as a server, not a relay; relay is used when the server is on a different subnet.
- ✗
The 'ipv6 unicast-routing' command is missing globally.
Why it's wrong here
Incorrect because IPv6 unicast routing is not required for the DHCPv6 server to function; it only affects routing protocol operation.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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