Simple definition
A DHCP Relay allows DHCP clients on one subnet to use a DHCP server located on another IP network.
Technical definition
Initial DHCP requests are broadcasts and normally do not cross a router. The relay receives them on the client-facing interface and forwards them to the DHCP server, usually by unicast, while providing information that identifies the originating subnet.
How it works / role
The client sends a DHCPDISCOVER. The relay receives it, sets relay information such as giaddr, and may add Option 82 data depending on the device. The server then selects the appropriate scope, replies to the relay, and the relay delivers the response back to the client VLAN.
What is it used for?
Centralize one or more DHCP servers while serving multiple VLANs or subnets.
Practical example
A FortiGate relays requests from VLAN 20 to a central DHCP server. The server uses the relay information to select the address pool for VLAN 20.
Common issues
- Incorrect DHCP server address
- UDP 67/68 is filtered between the relay and server
- No scope matches the relayed subnet
- Option 82 or relay policy is incompatible with the server
Related terms
Key takeaway: A DHCP relay does not allocate leases itself; it transports the exchange between a local client and a remote DHCP server.