mediumMultiple Choice
300-410 Practice Question: Router R6 has the following DHCPv6 configuration:…
Router R6 has the following DHCPv6 configuration:
ipv6 dhcp pool DHCP6_POOL3 address prefix 2001:db8:3::/64 lifetime 3600 600 dns-server 2001:db8::1 !
interface GigabitEthernet0/2
ipv6 address 2001:db8:3::1/64 ipv6 dhcp server DHCP6_POOL3 ipv6 nd managed-config-flag
no shutdown
What is the effect of the lifetime parameters 3600 and 600?
⚠ Common exam trap
The trap here is that Cisco tests whether candidates know the correct order of the preferred and valid lifetime parameters in the `lifetime` command, as many mistakenly assume the valid lifetime comes first or that the router will accept an invalid lifetime relationship.
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 preferred lifetime is 3600 seconds and the valid lifetime is 600 seconds, which is invalid because the valid lifetime must be greater than or equal to the preferred lifetime.
In the `ipv6 dhcp pool` configuration, the `lifetime` command specifies the preferred lifetime first (3600 seconds) and the valid lifetime second (600 seconds). According to RFC 4862, the valid lifetime must be greater than or equal to the preferred lifetime; otherwise, the configuration is invalid and will be rejected by the router. This mismatch causes the DHCPv6 pool to fail to apply the prefix.
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 preferred lifetime is 3600 seconds and the valid lifetime is 600 seconds, which is invalid because the valid lifetime must be greater than or equal to the preferred lifetime.
Why this is correct
In IPv6, the preferred lifetime and valid lifetime are configured in that order, so the command specifies preferred=3600 seconds and valid=600 seconds. RFC 4862 mandates that the valid lifetime must be greater than or equal to the preferred lifetime, because a prefix should remain valid at least as long as it is preferred. Here the preferred lifetime exceeds the valid lifetime, which makes the configuration invalid and will be rejected by the router.
- ✗
The preferred lifetime is 600 seconds and the valid lifetime is 3600 seconds, which is a typical configuration.
Why it's wrong here
This answer incorrectly reverses the two lifetime values: the configured order is preferred first, then valid, so the actual values are preferred=3600 and valid=600, not the opposite. While a configuration of preferred=600 and valid=3600 would technically satisfy RFC 4862, that is not what is present in the command, so this option misstates the facts and wrongly treats the invalid configuration as typical.
- ✗
The lifetimes are applied to the DNS server address, not the address prefix.
Why it's wrong here
Lifetimes in DHCPv6 are attributes of the assigned IPv6 address prefix, controlling how long the client may use the prefix and when it should prefer a new one. DNS server addresses are carried as a separate DHCPv6 option (e.g., option 23) and do not have preferred or valid lifetimes associated with them, so stating that the lifetimes apply to the DNS server is fundamentally incorrect.
- ✗
The configuration is valid and will work as expected.
Why it's wrong here
This configuration is not valid and will not operate as expected because the valid lifetime (600 seconds) is shorter than the preferred lifetime (3600 seconds). Per RFC 4862 and Cisco's DHCPv6 implementation, the valid lifetime must be greater than or equal to the preferred lifetime, and a violation of this requirement causes the configuration to be rejected. Therefore, this option incorrectly claims the configuration is workable.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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