CompTIA Network+ (N10-009)Network OperationsHard
A company's DHCP server resides on VLAN 10, but clients on VLAN 20 are unable to obtain IP addresses from it because the router separates the broadcast domains. Which configuration on the VLAN 20 interface will resolve this issue?
- ADHCP scope exclusion range
- BDHCP reservation
- CDHCP snooping
- DIP helper address (DHCP relay)
Show answer & explanationAnswer & explanation
Correct answer: D. IP helper address (DHCP relay)
Because DHCP discovery messages are broadcasts that do not cross routed boundaries, an IP helper address (DHCP relay agent) must be configured on the VLAN 20 interface to forward client broadcasts as unicast requests to the DHCP server on VLAN 10.
Why the other options are wrong
- A. An exclusion range removes addresses from a scope and has no effect on cross-subnet request forwarding.
- B. A DHCP reservation assigns a specific IP to a known MAC address but does not solve cross-subnet delivery.
- C. DHCP snooping is a switch security feature that filters rogue DHCP servers, not a relay function.
DHCP Relay (IP Helper)
A router feature that forwards DHCP broadcast messages from clients on one subnet to a DHCP server on a different subnet by converting them to unicast.
- Needed because DHCP discovery is a Layer 2 broadcast
- Configured on the router interface facing the client subnet
- Also called 'ip helper-address' in Cisco IOS
Memory trick: Broadcasts can't cross bridges — relay 'helps' them over.