hardMultiple ChoiceObjective-mapped
Fix DHCPv6 Guard Blocking Server Advertise/Reply Messages on Untrusted Port
An engineer is troubleshooting a network where IPv6 hosts cannot obtain IP addresses via DHCPv6. The switch is configured with DHCPv6 Guard to prevent rogue DHCP servers. The legitimate DHCPv6 server is connected to port GigabitEthernet1/0/1. The engineer sees that DHCPv6 Solicit messages from hosts reach the server, but the server's Advertise and Reply messages are not reaching the hosts. What is the most likely root cause?
Quick Answer
The answer is that the port connected to the legitimate DHCPv6 server is not configured as a trusted port for DHCPv6 server messages. This is because DHCPv6 Guard, when applied globally, drops all DHCPv6 server-originated messages—such as Advertise and Reply—on any port that is not explicitly trusted, even if the server is legitimate. The Solicit messages from hosts reach the server because they are client messages, which are allowed by default on untrusted ports, but the server’s responses are blocked at the switch port. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of DHCPv6 Guard’s asymmetric filtering: it only filters server-to-client traffic on untrusted ports, not client-to-server traffic. A common trap is assuming that because Solicit messages pass, the server is reachable, but the guard policy specifically targets the return path. Remember the memory tip: “Solicit goes through, but Advertise and Reply are denied—trust the server port to let the server speak.”
⚠ Common exam trap
Cisco often tests the distinction between DHCPv6 Guard (which blocks server messages) and other IPv6 First Hop Security features like RA Guard or IPv6 Source Guard, causing candidates to confuse the specific message types each feature filters.
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 Guard policy is applied globally, and the port connected to the DHCP server is not configured as a trusted port for DHCPv6 server messages.
DHCPv6 Guard is a First Hop Security feature that blocks DHCPv6 server messages (Advertise, Reply) on untrusted ports. When the policy is applied globally, all ports are untrusted by default unless explicitly configured as trusted. Since the legitimate DHCPv6 server is connected to GigabitEthernet1/0/1, that port must be configured as a trusted port using the `ipv6 dhcp guard trust` interface command. Without this, the switch drops the server's Advertise and Reply messages, preventing hosts from receiving valid DHCPv6 responses.
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 DHCPv6 Guard policy is applied globally, and the port connected to the DHCP server is not configured as a trusted port for DHCPv6 server messages.
Why this is correct
Correct because DHCPv6 Guard by default blocks server messages on untrusted ports; the server port must be explicitly trusted.
- ✗
RA Guard is blocking the DHCPv6 server's Router Advertisements, causing hosts to not send Solicit messages.
Why it's wrong here
Incorrect because the symptom states Solicit messages reach the server, so RA Guard is not the issue.
- ✗
IPv6 Source Guard is filtering the server's responses because the server's IPv6 address is not in the binding table.
Why it's wrong here
Incorrect because Source Guard filters based on source address of traffic from hosts, not from servers; also, DHCPv6 server messages are typically not filtered by Source Guard.
- ✗
The switch has DHCP snooping enabled for IPv4, which is interfering with IPv6 DHCPv6 operation.
Why it's wrong here
Incorrect because IPv4 DHCP snooping does not affect IPv6 DHCPv6; they are separate processes.
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Same concept, more angles
3 more ways this is tested on 300-410
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. In IPv6 First Hop Security, what is the purpose of the 'device-role' command in a DHCP guard policy?
medium- ✓ A.It specifies whether the interface is a server, client, or relay for DHCP filtering.
- B.It sets the trust level for ND inspection.
- C.It defines the VLAN membership for the interface.
- D.It enables IPv6 routing on the interface.
Why A: The 'device-role' command in a DHCP guard policy specifies the role of the interface as either a DHCP server, client, or relay. This allows the switch to filter DHCP messages based on the expected role, blocking unauthorized DHCP server messages on interfaces that should only have clients or relays. It is a key component of IPv6 First Hop Security to prevent rogue DHCPv6 servers.
Variation 2. A network engineer runs the following command to troubleshoot DHCPv6 guard: R1# debug ipv6 dhcp guard *Mar 1 00:03:45.678: IPv6-DHCP-Guard: R1, Fa0/0, DHCPv6 SOLICIT from fe80::3, client DUID 00010001abcd1234 *Mar 1 00:03:45.678: IPv6-DHCP-Guard: R1, Fa0/0, DHCPv6 SOLICIT from fe80::3 is allowed by policy DHCP-POLICY *Mar 1 00:03:46.901: IPv6-DHCP-Guard: R1, Fa0/0, DHCPv6 ADVERTISE from fe80::4, server DUID 0001000156789012 *Mar 1 00:03:46.901: IPv6-DHCP-Guard: R1, Fa0/0, DHCPv6 ADVERTISE from fe80::4 is blocked by policy DHCP-POLICY What does this output indicate?
medium- ✓ A.DHCPv6 guard is allowing client messages but blocking server messages from untrusted sources, preventing rogue DHCPv6 servers.
- B.DHCPv6 guard is blocking all DHCPv6 messages, indicating a misconfiguration.
- C.DHCPv6 guard is allowing all messages but logging them for analysis.
- D.DHCPv6 guard is not configured; the debug output is from default DHCPv6 behavior.
Why A: The debug output shows that DHCPv6 SOLICIT messages from client fe80::3 are allowed by policy DHCP-POLICY, while DHCPv6 ADVERTISE messages from server fe80::4 are blocked by the same policy. This is the expected behavior of DHCPv6 guard: it permits client messages (SOLICIT, REQUEST, etc.) to reach potential servers, but it blocks server messages (ADVERTISE, REPLY, etc.) from untrusted ports to prevent rogue DHCPv6 servers from assigning malicious configurations. Option A correctly identifies this selective filtering.
Variation 3. A network engineer runs the following command to verify DHCPv6 guard policy: R1# show ipv6 dhcp guard policy DHCP-POLICY Policy: DHCP-POLICY Status: Active Device role: dhcp-client Trusted ports: none Untrusted ports: Fa0/0 DHCPv6 guard: enabled DHCPv6 guard action: block DHCPv6 server validation: enabled DHCPv6 server list: 2001:db8::10 What does this output indicate?
medium- ✓ A.The policy blocks DHCPv6 server messages on Fa0/0 except from server 2001:db8::10.
- B.The policy allows all DHCPv6 messages on Fa0/0 without any filtering.
- C.The policy only applies to DHCPv6 client messages and ignores server messages.
- D.The policy is inactive and not applied to any interface.
Why A: The output shows that DHCPv6 guard is enabled with an action of 'block' on untrusted port Fa0/0, and DHCPv6 server validation is enabled with a server list containing 2001:db8::10. This means the policy blocks DHCPv6 server messages (e.g., ADVERTISE, REPLY) received on Fa0/0, but allows them if they originate from the specified server address. Thus, only server messages from 2001:db8::10 are permitted, while all other DHCPv6 server messages are blocked.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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