DHCP on a Routed Network
Since DHCP uses broadcasts, and routers block broadcasts, clients on remote subnets may be unable to receive TCP/IP information from a DHCP Server. Clients will use Automatic Private IP Addressing (APIPA) if they are unable to locate a DHCP server. Using APIPA a client will assign itself a random address between 169.254.0.1 – 169.254.254.254/16, allowing it to communicate with other clients on the same network segment who are also using APIPA.
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In order for clients on remote subnets to use DHCP, A DHCP relay-agent will need to be configured. The DHCP relay-agent listens for DHCP Traffic on its local network and forwards it to a DHCP Server. The DHCP Client can then get TCP/IP information from the DHCP Server.
To configure a DHCP relay agent, routing and remote access will need to be started. Click on Start. Select Administrative Tools > Routing and Remote Access.
The Routing and Remote Access management console will appear, expand the server. The red arrow next to the server indicates that the service is not running. To start the service right-click on the server. Select Configure and Enable Routing and Remote Access. The Routing and Remote Access Server Setup Wizard will appear. Click on Next to continue. Select Custom configuration. Click on Next to continue. Select LAN routing. Click on Next to continue. Click on Finish to close the Routing and Remote Access Server wizard. Routing and Remote Access has been installed. Click on Yes to start the service. The service will then start.
The green arrow next to the server indicates that the service has started. To install the DHCP relay agent, expand IP routing. To add the DHCP relay agent protocol, right-click on General. Select new Routing Protocol. A list of available routing protocols is displayed. Ensure that DHCP Relay Agent is highlighted and click on OK.
The DHCP Relay Agent has been installed. To configure the new Relay Agent, right-click on DHCP Relay Agent. Select New Interface. Choose the interface that the DHCP Relay Agent is to run on – in this case, Local Area Connection. Click on OK, to configure the DHCP Relay Agent to use the Local Area Connection interface. Ensure that the Relay DHCP Packets check box is selected. The Hop-count threshold indicates the maximum number of DHCP relay agents that will handle DHCP relayed traffic. The Boot threshold specifies how many seconds the relay agent waits before forwarding DHCP messages. This option is useful if you want a local DHCP server to respond first.
If the local DHCP server doesn’t respond then the packet will be forwarded to a remote DHCP server. Click on OK to continue. To add the address of the remote DHCP server, right-click on DHCP Relay Agent. Select Properties. Type the IP address of the DHCP Server into the Server address box. Click Add. Click on OK to close the Properties dialog box. The DHCP Relay Agent has been installed and configured.