300-440 Question 273
Single answerAn enterprise network has a DHCP server configured on a Cisco IOS router. IPv4 DHCP clients on a remote subnet are not receiving IP addresses, but the DHCP server is functional for clients on the same subnet. A DHCP relay agent is configured on the Layer 3 interface for the remote subnet. What is the most likely cause of the issue?
- A
The DHCP relay agent is missing the 'ip helper-address' command pointing to the DHCP server's IP address.
- B
The DHCP server has run out of available IP addresses in the DHCP pool.
- C
The DHCP relay agent is not configured to forward IPv4 broadcast traffic.
- D
The DHCP options configured on the server are incompatible with the client devices.
Show answer and explanation
Correct answer: A
Explanation
When using a DHCP relay agent, the 'ip helper-address' command is essential to forward DHCP requests from remote clients to the DHCP server. Without this command, the router does not know where to send the client's DHCP broadcast traffic, which prevents remote clients from receiving IP addresses. This issue would not affect clients in the same subnet as the DHCP server, as they can communicate directly with it.
- A. Correct.
The 'ip helper-address' command is required on the interface of the Layer 3 device acting as the relay agent. Without it, the DHCP broadcast packets from clients will not be forwarded to the DHCP server.
- B. Incorrect.
While this could cause DHCP failures, it would affect all clients, not just those on the remote subnet. The issue is isolated to the relay agent configuration in this scenario.
- C. Incorrect.
DHCP relay agents do not forward broadcast traffic by default, but they forward DHCP packets as unicast to the server when the 'ip helper-address' command is configured.
- D. Incorrect.
DHCP options being incompatible would typically cause issues with certain features or services but not prevent IP address assignment altogether.