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350-401 Practice Question: Is troubleshooting a DHCP issue on a Cisco router…
A network engineer is troubleshooting a DHCP issue on a Cisco router configured as a DHCP server for a VLAN. Clients in the VLAN are able to obtain IP addresses from the DHCP server, but they are not receiving the correct DNS server address. The engineer checks the DHCP pool configuration and sees the dns-server command is configured with the correct IP address. What is the most likely cause of the problem?
⚠ Common exam trap
Cisco often tests the misconception that configuring the `dns-server` command alone is sufficient, when in fact the pool must be correctly bound to the client's subnet via the `network` command for any options to be applied.
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 DHCP pool is not associated with the correct VLAN interface using the network command.
The most likely cause is that the DHCP pool is not associated with the correct VLAN interface via the `network` command. Even if the `dns-server` command is correctly configured, the router will not apply that pool to clients on a different subnet. The DHCP server uses the `network` statement to match the client's subnet (derived from the interface IP) to the correct pool; if the pool's network does not match the VLAN interface's subnet, clients will receive an address from a different pool (or no pool) that lacks the correct DNS server.
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 DHCP pool is not associated with the correct VLAN interface using the network command.
Why this is correct
The DHCP pool is correctly identified as the root cause when the network command defines a subnet that does not match the VLAN interface's primary IP subnet. Cisco IOS selects a DHCP pool by comparing the network statement's subnet with the subnet of the interface that received the client's DHCP request (or the giaddr from a DHCP relay). If there is no matching pool, the server may assign an address from a different, incorrectly configured pool or fail to offer an address entirely, and critically, pool-specific options such as the DNS server are not delivered because they are bound to the pool that is actually selected. The symptom of clients obtaining an IP but no DNS option strongly points to this mismatch.
- ✗
The DNS server is unreachable from the DHCP server.
Why it's wrong here
The reachability of the DNS server from the DHCP server is entirely irrelevant to whether the DNS option is included in a DHCP offer. DHCP is a broadcast/unicast protocol that simply transports configuration options as data within the packet; the DHCP server does not validate or contact the DNS server before sending its IP address in the offer. The server only needs the DNS server's IP address manually configured in the pool (via the dns-server option). A DNS server that is unreachable from the DHCP server would still be successfully offered to clients—they just wouldn't be able to use it, which is a different failure mode.
- ✗
The ip dhcp excluded-address command is blocking the DNS server IP.
Why it's wrong here
The ip dhcp excluded-address command is used to reserve IP addresses from the DHCP pool so they are never offered to clients as leaseable addresses—for example, to protect static addresses of servers or routers. This command has no effect on option delivery: options like the DNS server are attached to the lease offer as configuration parameters, not as addresses assigned from the pool. Blocking a DNS server's IP with excluded-address would only prevent that IP from being leased to a client, which is not the problem here, since the DNS server is a separate host and its address is being passed as an option, not counted against the pool's available addresses.
- ✗
The DHCP client is configured with a static DNS server address.
Why it's wrong here
If a DHCP client has a statically configured DNS server address, it will indeed ignore the DNS server option received from the DHCP server, because local static configuration typically overrides DHCP-provided settings. However, this scenario implies the client is using DHCP for all of its IP configuration, including DNS, as evidenced by the problem description focusing on a DHCP-provided option. Moreover, even with a static DNS, the client would still receive and process other DHCP options; the absence of the DNS option in the offer would indicate a server-side pool misconfiguration, not client-side overrides. The question's context points to the server failing to send the option, not the client choosing to ignore it.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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