N10-009 Question 228
Single answerA company has a centralized DHCP server in the data center for all branch offices. At one branch, users on VLAN 20 are not receiving IPv4 addresses, but they can reach the default gateway on their local subnet. Devices on other VLANs at the branch are receiving addresses normally. The DHCP scope for VLAN 20 exists on the central server and has available addresses. The network administrator confirms there is no local DHCP server on VLAN 20. Which of the following is the MOST likely cause of the issue?
- A
The DHCP scope for VLAN 20 is missing an exclusion range for statically assigned devices
- B
The router interface for VLAN 20 is missing a DHCP relay/IP helper configuration
- C
The DHCP lease time for VLAN 20 is set too long, preventing new clients from requesting addresses
- D
The clients on VLAN 20 should be using SLAAC instead of DHCP for IPv4 addressing
Show answer and explanation
Correct answer: B
Explanation
The key troubleshooting clue is that the affected clients are on a different subnet from the centralized DHCP server, they can reach their local default gateway, and the DHCP scope exists with free addresses. DHCP for IPv4 relies on client broadcasts such as DHCPDISCOVER, which are not routed across VLANs or subnets unless a DHCP relay agent forwards them to the remote server. On many enterprise routers and Layer 3 switches, this is implemented with an IP helper-address or equivalent DHCP relay configuration on the SVI or routed interface for that VLAN. Since other VLANs at the same branch are working, the problem is most likely specific to VLAN 20's relay configuration rather than a server-wide issue. This aligns with common vendor documentation and best practices for centralized DHCP deployments: define the correct scope on the server, ensure the default gateway address is provided in DHCP options as needed, and configure DHCP relay on each routed interface that serves DHCP clients on remote subnets.
- A. Incorrect.
This is incorrect. Missing exclusions can cause address conflicts if statically assigned addresses overlap with the DHCP pool, but it would not typically prevent all clients on a specific VLAN from obtaining leases when the scope exists and has available addresses. Clients would still be able to receive offers from the DHCP server unless there was a separate relay or reachability issue.
- B. Correct.
This is correct. DHCP Discover messages are broadcast traffic, and routers do not forward broadcasts between subnets by default. In a design with a centralized DHCP server serving remote VLANs, the Layer 3 interface for that VLAN must be configured with DHCP relay functionality, commonly called an IP helper on many platforms. If only VLAN 20 is affected while other VLANs work, the most likely explanation is that the relay/helper is missing or misconfigured specifically on the VLAN 20 interface.
- C. Incorrect.
This is incorrect. A long lease time can slow address turnover in a small pool and potentially contribute to address exhaustion, but the scenario explicitly states the scope has available addresses. Lease time does not stop new clients from sending DHCP Discover requests or the server from offering available addresses.
- D. Incorrect.
This is incorrect. SLAAC is used for IPv6 stateless address autoconfiguration, not for IPv4 addressing. IPv4 clients typically use DHCP, APIPA, or static configuration. Choosing SLAAC here reflects a common misconception of applying IPv6 concepts to an IPv4 problem.