300-410 Infrastructure Services Practice Question
A network engineer is configuring a Cisco IOS XE router as a DHCPv6 server for a dual-stack network. The router must provide IPv6 addresses and other configuration parameters to clients on VLAN 20. The engineer has configured a DHCPv6 pool named POOL1 with the address prefix 2001:DB8:20::/64 and the DNS server 2001:DB8::53. The clients are not receiving IPv6 addresses. Which additional configuration is required on the router's VLAN 20 interface to ensure DHCPv6 clients can obtain addresses?
⚠ Common exam trap
Many candidates confuse the router's role as a DHCPv6 server with a DHCPv6 client; the ipv6 address dhcp command is for client operation, not for serving addresses.
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
✓
ipv6 dhcp server POOL1
To make a Cisco IOS XE router act as a DHCPv6 server on an interface, you must bind the DHCPv6 pool to that interface using the ipv6 dhcp server command. The pool configuration alone defines the parameters, but the interface must be explicitly enabled to serve DHCPv6. Without this command, the router will not listen for or respond to DHCPv6 client requests on VLAN 20. Therefore, the correct answer is the interface-level command that activates the server function.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
ipv6 address dhcp
Why it's wrong here
The ipv6 address dhcp command configures the interface itself as a DHCPv6 client to obtain an address, which is not the goal here. The router is acting as a DHCPv6 server, not a client. This command would be used on a host interface to acquire an address from an upstream DHCPv6 server. It does not enable the router to serve addresses to downstream clients on VLAN 20. Therefore, it is incorrect for this scenario.
- ✗
ipv6 nd managed-config-flag
Why it's wrong here
The ipv6 nd managed-config-flag command sets the M flag in Router Advertisement messages, indicating that clients should use DHCPv6 to obtain addresses. While this is often used with DHCPv6, it is not strictly required for the DHCPv6 server to function; clients can still be configured to use DHCPv6 manually. Moreover, without the ipv6 dhcp server command, the router will not respond to DHCPv6 requests at all. This command alone does not enable the DHCPv6 server function.
- ✓
ipv6 dhcp server POOL1
Why this is correct
To enable the router to act as a DHCPv6 server on an interface, the ipv6 dhcp server <pool-name> command must be configured on that interface. This binds the DHCPv6 pool to the interface and allows the router to respond to DHCPv6 solicit messages from clients. Without this command, the router will not process DHCPv6 requests on VLAN 20, even though the pool exists. This is the missing piece to make the server operational.
- ✗
ipv6 nd other-config-flag
Why it's wrong here
The ipv6 nd other-config-flag command sets the O flag in Router Advertisements, telling clients to use DHCPv6 to obtain other configuration parameters such as DNS servers, but not addresses. This is useful when addresses are assigned via SLAAC but other parameters come from DHCPv6. In this scenario, the goal is to provide IPv6 addresses via DHCPv6, so the O flag is not sufficient. Additionally, the DHCPv6 server must be enabled on the interface with the ipv6 dhcp server command.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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