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300-410 Practice Question: A large enterprise network is experiencing…
A large enterprise network is experiencing intermittent IPv6 connectivity loss for hosts on VLAN 100. Router R1 has the following relevant configuration:
interface GigabitEthernet0/0.100 encapsulation dot1Q 100
ipv6 address 2001:DB8:1:100::1/64 ipv6 nd raguard ipv6 nd prefix default ipv6 dhcp relay destination 2001:DB8:1:200::1 !
Router R2 shows: debug ipv6 dhcp relay output indicates that DHCPv6 requests from VLAN 100 are being relayed, but the server never receives the SOLICIT messages. What is the root cause?
⚠ Common exam trap
Many candidates confuse the 'ipv6 nd raguard' command with a DHCPv6 filter, when in reality it only applies to Router Advertisement messages, not DHCPv6 packets.
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
✓
An IPv6 ACL applied to GigabitEthernet0/0.100 has an implicit deny that blocks the relayed DHCPv6 traffic.
An IPv6 ACL applied to the interface can implicitly deny the relayed DHCPv6 traffic. The debug output shows that DHCPv6 requests are being relayed from R1, but the server never receives the SOLICIT messages, indicating that the relayed packets are being dropped before leaving the router. An implicit deny in an IPv6 ACL on the outgoing interface (or inbound on the next hop) would block the relayed DHCPv6 unicast traffic, while the local relay process still sees the original client messages.
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 'ipv6 nd raguard' command on the interface filters DHCPv6 SOLICIT messages, preventing relay.
Why it's wrong here
RA Guard filters Router Advertisements, not DHCPv6 messages.
- ✗
The DHCPv6 relay destination is in a different VRF, and the relay is not configured to use that VRF.
Why it's wrong here
No VRF configuration is shown; the relay destination is reachable via the global table.
- ✓
An IPv6 ACL applied to GigabitEthernet0/0.100 has an implicit deny that blocks the relayed DHCPv6 traffic.
Why this is correct
The implicit deny at the end of an IPv6 ACL can block DHCPv6 relay packets if no explicit permit statement exists for the relay destination.
- ✗
The DHCPv6 server is not configured to accept relayed messages from this relay agent.
Why it's wrong here
DHCPv6 servers accept relayed messages by default; the issue is on the relay side.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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