N10-009 Question 227
Single answerA company has centralized its DHCP service on a server in the data center. A new branch office subnet, 192.168.40.0/24, was recently added behind a Layer 3 router. Users at the branch report that they are not receiving IPv4 addresses automatically, while devices on other subnets work normally. The DHCP scope for 192.168.40.0/24 already exists on the server and is active. Which action should the network administrator take to allow clients on the new subnet to obtain IPv4 addresses from the centralized DHCP server?
- A
Configure a DHCP relay/IP helper on the branch router interface for 192.168.40.0/24
- B
Enable SLAAC on the branch router so IPv4 clients can self-assign addresses from the DHCP scope
- C
Create DHCP reservations for all branch client MAC addresses on the DHCP server
- D
Reduce the lease time on the existing 192.168.40.0/24 scope so addresses are offered more quickly
Show answer and explanation
Correct answer: A
Explanation
The issue is classic cross-subnet DHCP behavior. DHCP clients initially send a broadcast DHCPDISCOVER, but routers do not forward Layer 2 broadcasts between subnets. In environments with a centralized DHCP server, each routed client subnet needs a DHCP relay agent configured on the gateway interface so those broadcasts can be forwarded as unicasts to the DHCP server. On many routers and Layer 3 switches, this is implemented with an IP helper address. Because the scope already exists and is active, the missing relay configuration is the most likely cause. Reservations, exclusions, scope settings, and lease times all operate on the server side after requests reach the DHCP service; they do not solve a failure to traverse a router. For IPv6, a comparable host-autoconfiguration concept is SLAAC, but that is separate from IPv4 DHCP. This aligns with common vendor documentation and best practices from Cisco and Microsoft for DHCP relay deployments in routed networks.
- A. Correct.
Correct. DHCP Discover messages are broadcast traffic, and routers do not forward broadcasts by default. When clients are on a different subnet than the DHCP server, the router or Layer 3 interface for that subnet must be configured as a DHCP relay agent, often called an IP helper. The relay converts the client broadcast into a unicast message to the DHCP server, allowing the server to offer addresses from the correct scope.
- B. Incorrect.
Incorrect. SLAAC is an IPv6 mechanism, not an IPv4 DHCP replacement. It uses ICMPv6 router advertisements so hosts can generate their own IPv6 addresses. It does not allow IPv4 clients to obtain addresses from an IPv4 DHCP scope.
- C. Incorrect.
Incorrect. Reservations are used to ensure specific devices receive consistent IP addresses based on MAC address, but they do not solve the problem of DHCP requests not reaching the server across a router. Even with reservations configured, the branch clients still need a relay path to the DHCP server.
- D. Incorrect.
Incorrect. Lease time affects how long an address assignment remains valid before renewal. It does not address the underlying issue that DHCP broadcast traffic from the new subnet is not reaching the remote DHCP server. Shortening the lease might be useful in high-churn environments, but it will not restore service in this scenario.