Question 634 of 1,389
CCNA Network Services and Security Practice Question
A network engineer has enabled DHCP snooping on a Catalyst switch to prevent rogue DHCP servers. All access ports in VLAN 10 are untrusted. A router attached to a trunk port on the switch acts as the default gateway for VLAN 10 and is configured with the ip helper-address 10.1.2.5, which points to a remote DHCP server. After enabling DHCP snooping, hosts in VLAN 10 cannot obtain IP addresses; packet captures show DHCPDISCOVER messages are sent, but no DHCPOFFER is received. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction that DHCP snooping blocks DHCP server messages (OFFER/ACK/NAK) on untrusted ports, not client messages (DISCOVER/REQUEST), leading candidates to incorrectly assume client messages are dropped.
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 switch port connecting to the router is not configured as a trusted port for DHCP snooping.
DHCP snooping treats all ports as untrusted by default. When a router acting as a DHCP relay is connected to an untrusted trunk port, the switch drops DHCPOFFER messages received from the router because they originate from an untrusted interface. Configuring the trunk port as trusted allows DHCP server responses (OFFER, ACK) to pass through to clients.
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's ip helper-address command is pointing to an incorrect DHCP server IP address.
Why it's wrong here
The ip helper-address configuration on the router is actually correct: it is set to 10.1.2.5, the remote DHCP server's address, and the scenario gives no indication of a typo or mispointed relay. A wrong helper-address would either prevent DHCPDISCOVER from being forwarded or send it to an unreachable/incorrect server, but the failure here occurs after the DISCOVER is relayed—the OFFER is generated and sent back but then dropped by the switch. Since the explicit configuration is correct, the root cause must lie elsewhere, namely in the switch's DHCP snooping trust state on the port facing the router.
- ✓
The switch port connecting to the router is not configured as a trusted port for DHCP snooping.
Why this is correct
DHCP snooping trusts only designated ports to forward DHCP server messages. Since the router relays the DHCPOFFER onto the trunk port, an untrusted port will cause the switch to discard the offer, resulting in DHCP failure.
- ✗
The DHCP server is on a different subnet, so the switch needs a switched virtual interface (SVI) in VLAN 10 with an IP address for Layer 3 connectivity.
Why it's wrong here
DHCP relay is performed by the router, not the switch. The switch does not need an SVI in the client VLAN because the router's subinterface (or routed port) handles inter-VLAN forwarding. The DHCP messages are already routed by the relay agent.
- ✗
DHCP snooping is dropping DHCPDISCOVER messages because the client access ports are untrusted.
Why it's wrong here
DHCP snooping does not drop DHCPDISCOVER or DHCPREQUEST messages on untrusted client-facing ports; that is their normal, expected behavior—clients are always untrusted and must be allowed to send discovery messages to request an address. What DHCP snooping filters is DHCP server-originated messages (OFFER, ACK, NAK) arriving on untrusted ports, because those indicate a rogue server. The actual issue is the opposite direction: the switch port connecting to the router (the relay agent) is untrusted, so the switch discards the valid DHCPOFFER that the router relays back into the VLAN, causing the client to never receive an offer.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The switch port connecting to the router is not configured as a trusted port for DHCP snooping.Correct answer▾
Why this is correct
DHCP snooping trusts only designated ports to forward DHCP server messages. Since the router relays the DHCPOFFER onto the trunk port, an untrusted port will cause the switch to discard the offer, resulting in DHCP failure.
✗The router's ip helper-address command is pointing to an incorrect DHCP server IP address.Wrong answer — click to see why▾
Why this is wrong here
This distractor exploits the common tendency to blame the helper address configuration first, overlooking the security feature that silently drops the returning DHCPOFFER.
✗The DHCP server is on a different subnet, so the switch needs a switched virtual interface (SVI) in VLAN 10 with an IP address for Layer 3 connectivity.Wrong answer — click to see why▾
Why this is wrong here
This plays on the misconception that a switch requires an IP address on the client VLAN to facilitate DHCP, when in fact the router acting as the relay agent provides Layer 3 connectivity.
✗DHCP snooping is dropping DHCPDISCOVER messages because the client access ports are untrusted.Wrong answer — click to see why▾
Why this is wrong here
This misinterprets DHCP snooping behavior: it assumes all DHCP traffic is filtered on untrusted ports, overlooking the critical distinction that only server-side messages are blocked, not client requests.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
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Last reviewed: Jun 25, 2026
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