DHCP Snooping Trusted Port Configuration
Your company has deployed a Cisco Catalyst 9300 switch stack as the distribution layer for a campus network. The network uses VLANs 10 (data), 20 (voice), and 30 (management). The switch stack is configured with DHCP snooping, Dynamic ARP Inspection (DAI), and IP Source Guard (IPSG) on access ports. Recently, users in VLAN 10 report intermittent connectivity issues. You notice that some users receive duplicate IP addresses from the DHCP server. The DHCP server is connected to a trunk port on the switch stack. After reviewing logs, you see that DHCPACK messages are being dropped on the trunk port. The DHCP snooping binding table shows entries for legitimate clients, but also some entries with MAC addresses from a different vendor. Which action should you take to resolve the issue?
Quick Answer
The answer is to configure the trunk port connecting to the DHCP server as a trusted port for DHCP snooping. This is correct because DHCP snooping, by default, treats all switch ports as untrusted, meaning it drops all DHCP server-originated messages like DHCPOFFER, DHCPACK, and DHCPNAK on those ports. Since the DHCP server is sending its acknowledgments through a trunk port, those DHCPACK messages are being filtered out, preventing clients from receiving their assigned IP addresses and causing the duplicate IP issue. On the ENCOR 350-401 exam, this scenario tests your understanding of the DHCP snooping trust boundary—a common trap is to forget that even trunk ports are untrusted by default, so you must explicitly mark the port facing the legitimate server with the `ip dhcp snooping trust` command. A solid memory tip is to think of the DHCP server as the only source of truth: trust the port it sits on, and everything else stays untrusted.
⚠ Common exam trap
Cisco often tests the default untrusted behavior of DHCP snooping on all ports, tricking candidates into thinking that only access ports need trust configuration, when in fact the port facing the DHCP server (even a trunk) must be explicitly trusted to allow server messages through.
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
✓
Configure the trunk port connecting to the DHCP server as a trusted port for DHCP snooping.
The DHCP snooping feature treats all ports as untrusted by default, which means DHCP server messages (DHCPOFFER, DHCPACK, DHCPNAK) are dropped on untrusted ports. Since the DHCP server is connected to a trunk port and DHCPACK messages are being dropped, that trunk port must be explicitly configured as a trusted port for DHCP snooping using the 'ip dhcp snooping trust' interface command. This allows legitimate DHCP server responses to reach clients, resolving the duplicate IP address issue caused by clients not receiving their assigned addresses.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Manually shut down the access ports that have unknown MAC addresses in the binding table.
Why it's wrong here
This does not address the DHCP server response being dropped.
- ✗
Disable Dynamic ARP Inspection on VLAN 10.
Why it's wrong here
DAI validates ARP packets, not DHCP messages.
- ✓
Configure the trunk port connecting to the DHCP server as a trusted port for DHCP snooping.
Why this is correct
DHCP snooping drops DHCP server responses on untrusted ports.
- ✗
Disable IP Source Guard on all access ports in VLAN 10.
Why it's wrong here
IPSG filters IP traffic based on binding table, not DHCP messages.
Visual reference
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
Go deeper
Related to this question
Learn chapter
Network Architecture Fundamentals
Key term
Dynamic ARP Inspection
Dynamic ARP Inspection is a security feature that validates ARP packets on a network to prevent man-in-the-middle attacks by ensuring that only legitimate ARP messages are forwarded.
Key term
IP Source Guard
IP Source Guard is a network security feature that blocks IP address spoofing by verifying that each packet's source IP address matches an authorized binding assigned to that switch port.
About these practice questions
One of 1,175 original 350-401 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more way this is tested on 350-401
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. A network administrator is troubleshooting a DHCP snooping issue on a Cisco switch. The switch is configured with DHCP snooping globally and on VLAN 10. The trusted interface is GigabitEthernet0/1 connected to the DHCP server. However, clients on VLAN 10 are not receiving IP addresses from the DHCP server. What is the most likely cause?
hard- A.The switch has IP Source Guard enabled, blocking valid DHCP traffic.
- ✓ B.The interface GigabitEthernet0/1 is not configured as a trusted port for DHCP snooping.
- C.The DHCP server is on a different subnet and the switch lacks an IP helper address.
- D.The DHCP server is sending offers too quickly, exceeding the rate-limit on the switch.
Why B: The scenario states that DHCP snooping is configured globally and on VLAN 10, and that GigabitEthernet0/1 is connected to the DHCP server. However, for DHCP snooping to allow DHCP server messages (OFFER, ACK) to be forwarded, the interface connected to the legitimate DHCP server must be explicitly configured as a trusted port using the 'ip dhcp snooping trust' interface command. Without this, the switch treats all DHCP server responses as untrusted and drops them, preventing clients from receiving IP addresses.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.