The DHCP Relay service allows you to pass DHCP broadcast messages to network segments a client computer is not directly attached to. DHCP relaying can be used to share a single DHCP server across logical network segments that are separated by a firewall. The DHCP Relay service does not handle IP addresses. It sends unicast messages instead of broadcast messages.
DHCP Relay Agent Between Two LANs:
Before You Begin
If you are using both DHCP and a DHCP Relay on the same firewall, verify that both services are not using the same interface.
Configure the DHCP Relay Agent
- Go to NETWORK > DHCP Relay.
- Select Enable DHCP Relay.
From the Relay Interfaces list, select the network interfaces that are used by the DHCP relay agent to connect to the DHCP server and client networks. To add the interface, click + after each selection.
- In the DHCP Server IPs field, add the IP addresses of the DHCP servers. Click + after each entry.
- Enter the UDP Port the relay agent is listening on. Default: 67
- Enable Add Agent ID if you want the DHCP relay agent to add an Agent ID (AID) to the transmitted packets. An AID indicates that the data has been relayed.
- Enter the Max. DHCP packet Size in bytes. Default: 1400
- From the Agent ID Relay Policy list, select how your DHCP relay agent handles DHCP packets that are already flagged by an AID from another agent:
- Append – (default) Attach your AID to the existing AID.
- Replace – Replaces the existing AID with your AID.
- Forward – Passes DHCP packets without any modification.
- Discard – Discards DHCP packets that are already flagged by an AID.
- From the Agent ID Mismatch Policy list, select how your DHCP relay agent handles DHCP server replies that do not contain its AID:
- Discard – (default) Discards the DHCP packet.
Forward – Forwards the DHCP packet to the DHCP client.
- Enter the Max.Packet Hop Count to avoid infinite packet loops (default:
- Select Forward Unicast Packets if Bootstrap/BOOTP unicast messages should be forwarded by the DHCP relay.
Cascading DHCP Relay Agents
The DHCP relay agent is not designed for cascaded use. If you must configure multiple relay agents in a cascaded environment, do not specify the server-side interface of the cascaded ('border') relay agent in the configuration; otherwise, this will lead to conflicts. In this example, two client subnets are connected to DHCP relay agents 1 and 2. When you configure the relay agents, the interfaces listening to broadcast requests from the clients (eth1 and eth4) must be specified as relay interfaces. The server-side interface of relay agent 2 (eth5), which is connected to the DHCP server, must NOT be specified.
Cascading DHCP Relay Agents with Interfaces to be Configured: