DHCP requests are usually sent as a broadcast, which means that the request reaches all participants in its (sub)net (also known as broadcast domain), but none for which a routing process would have to be started.
DHCP cannot communicate across a network boundary due to the broadcast. As soon as the client network and the DHCP server are separated from each other, no IPs can be assigned because the requests, the DHCP discover, do not reach the server. If you separate your networks with VLANs, for example, you have to consider something like that. A DHCP server can be implemented on the router that connects the networks, but this is an unclean solution.
The most elegant solution to the problem is a DHCP Relay Agent, or IP Helper as Cisco calls it. The IP helper then has the function of converting the incoming broadcast into a unicast and taking over the communication with the DHCP server. The DHCP relay is implemented at the outermost layer3 component.
In the above example, the DHCP server is in network C and supplies it with IP addresses. The IPHelpers in network A and B take over the communication for the clients with the DHCP server.
The relay agent can also be implemented directly in the network, for example on a Windows server with routing and RAS services.
The DHCP server does not have to be directly connected to the router with the relay agent, but the network interface on which the IP Helper is running must be able to reach the DHCP server. So if there is a firewall in between, ports 67 and 68 UDP must also be opened. Of course you need a separate IP range for each network, otherwise the networks cannot communicate with each other.
Since Windows Server 2012 it has been very easy to cluster several DHCP servers together, which means that two or more DHCP servers share 100% of the address space and distribute the leases (IP addresses) in the networks.
Each server in the cluster needs its own IP helper in the other networks. You can also enter a network broadcast address as a relay agent, but for this the cluster must be in the same network and the IP Helper must be able to send a broadcast into the network.
The cluster can be configured as a failover or load balancer. If the load balancing mode is selected, the percentage distribution of the IPs per server must be configured with . With the cluster, you don't have to configure a division of the DHCP range for two servers in the same network.
