- A
The router has a static IPv6 address configured on the same interface, and the DHCP-assigned address is not preferred for source selection.
Correct: If a static address is present, the router may use it as the source, especially if it is in the same subnet as the destination.
- B
The DHCPv6 server assigned a temporary address (IA_TA) instead of a non-temporary address (IA_NA).
Why wrong: Incorrect: The 'ipv6 address dhcp' command requests IA_NA, not IA_TA.
- C
The router's 'ipv6 unicast-routing' is disabled.
Why wrong: Incorrect: Unicast routing is needed for forwarding, but the address would still be used.
- D
The DHCP-assigned address is in a different subnet than the default gateway.
Why wrong: Incorrect: The DHCP server typically assigns an address from the same subnet as the client.
Quick Answer
The answer is that a static IPv6 address configured on the same interface is preferred over the DHCPv6-assigned address for outgoing traffic. This occurs because the router’s source address selection logic prioritizes manually configured static addresses over dynamically learned ones, even when the DHCPv6 client successfully obtains a valid non-temporary address via the `ipv6 address dhcp` command. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of IPv6 source address selection rules and the interaction between static and dynamic addressing on a single interface. A common trap is assuming the DHCPv6 client address is automatically used for all outbound packets, but the router will only use it if no static global unicast address is present or if the static address is removed. Remember the memory tip: “Static beats dynamic for source selection”—the router always favors the manually configured address unless explicitly overridden.
300-410 DHCP (IPv4 and IPv6) Practice Question
This 300-410 practice question tests your understanding of dhcp (ipv4 and ipv6). This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A router is configured as a DHCPv6 client on an interface using 'ipv6 address dhcp'. The router receives a valid IPv6 address from the server, but the address is not used for outgoing traffic. Which is the most likely explanation?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 has a static IPv6 address configured on the same interface, and the DHCP-assigned address is not preferred for source selection.
When a router uses 'ipv6 address dhcp', it requests a non-temporary address (IANA). However, the router may also have a link-local address and possibly a static address. The source address selection for outgoing traffic is based on the destination and the routing table. If the router has a static global unicast address configured, it may prefer that over the DHCP-assigned address. A common edge case is that the DHCP-assigned address is not preferred due to the 'ipv6 dhcp client' command not setting the address as the primary.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 has a static IPv6 address configured on the same interface, and the DHCP-assigned address is not preferred for source selection.
Why this is correct
Correct: If a static address is present, the router may use it as the source, especially if it is in the same subnet as the destination.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The DHCPv6 server assigned a temporary address (IA_TA) instead of a non-temporary address (IA_NA).
Why it's wrong here
Incorrect: The 'ipv6 address dhcp' command requests IA_NA, not IA_TA.
- ✗
The router's 'ipv6 unicast-routing' is disabled.
Why it's wrong here
Incorrect: Unicast routing is needed for forwarding, but the address would still be used.
- ✗
The DHCP-assigned address is in a different subnet than the default gateway.
Why it's wrong here
Incorrect: The DHCP server typically assigns an address from the same subnet as the client.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Trap categories for this question
Command / output trap
Incorrect: The 'ipv6 address dhcp' command requests IA_NA, not IA_TA.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
DHCP (IPv4 and IPv6) — This question tests DHCP (IPv4 and IPv6) — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The router has a static IPv6 address configured on the same interface, and the DHCP-assigned address is not preferred for source selection. — When a router uses 'ipv6 address dhcp', it requests a non-temporary address (IANA). However, the router may also have a link-local address and possibly a static address. The source address selection for outgoing traffic is based on the destination and the routing table. If the router has a static global unicast address configured, it may prefer that over the DHCP-assigned address. A common edge case is that the DHCP-assigned address is not preferred due to the 'ipv6 dhcp client' command not setting the address as the primary.
What should I do if I get this 300-410 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jun 19, 2026
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