CompTIA Network+ (N10-009)Network TroubleshootingMedium
Workstations in Building A, which share the same subnet as the DHCP server, receive IP addresses normally. Workstations in Building B, located across a router on a different subnet, fail to receive DHCP-assigned addresses. Static IP addressing works fine in Building B. What is the most likely cause?
- AThe DHCP server's lease duration is set too short
- BBuilding B workstations have DNS servers misconfigured
- CThe DHCP server's scope has been exhausted
- DThe router interface facing Building B is missing a DHCP relay (IP helper) configuration
Show answer & explanationAnswer & explanation
Correct answer: D. The router interface facing Building B is missing a DHCP relay (IP helper) configuration
DHCP is a broadcast-based protocol, and broadcasts do not cross routers by default. A router must be configured with a DHCP relay agent (IP helper address) to forward DHCPDISCOVER broadcasts from a remote subnet to the DHCP server. Since Building A works and static addressing works in Building B, the DHCP server and physical connectivity are fine.
Why the other options are wrong
- A. A short lease time causes frequent renewals, not total failure to obtain an address.
- B. DNS misconfiguration wouldn't prevent DHCP address assignment.
- C. Scope exhaustion would also affect Building A eventually and wouldn't explain a subnet-specific failure.
DHCP Relay Agent (IP Helper)
A router configuration (ip helper-address) that forwards DHCP broadcast requests from a remote subnet to a centralized DHCP server.
- DHCP discover/request messages are broadcasts and don't cross routers by default.
- IP helper-address converts broadcasts to unicast toward the DHCP server.
- Missing relay config is a common cause of DHCP failure across subnets.
Memory trick: 'Broadcasts stop at the router — someone has to relay the message.'