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Understanding and Configuring the DHCP Relay Agent

Before we leave this discussion of routing, we should discuss issues related to running DHCP in environments where either remote clients (as discussed in the previous hour) receive their IP addresses from a DHCP server or you have a subnetted IP network that uses DCHP. Both these problem scenarios (in terms of getting IP addressing information to client computers) can be solved by deploying the DHCP Relay Agent.

In the case of routed networks, routers, by design, do not pass broadcast messages from one subnet to another, so this is a problem if you are using the DHCP service but do not have a DHCP server on each of the IP subnets. The DHCP Relay Agent can be configured so that it knows the location of a DHCP server or servers on other subnets of the network (that is, on the other side of the router). The Relay Agent takes the broadcast request from DHCP clients on the subnet (where both the client and the Relay Agent reside) and relays a point-to-point communication to the DHCP server that there has been a request for an IP address lease. (This type of communication is passed on by the router because it is not a broadcast message, but is directed to the specific IP address of the DHCP server.)

As already mentioned, the DHCP Relay Agent is also necessary when you are providing remote access clients with IP addresses from a DHCP server. The DHCP Relay Agent relays IP address requests from remote clients to the DHCP server.

You cannot set up the DHCP Relay Agent on a server that is a DHCP server, nor can you install it on a server that is running Network Address Translation (NAT is discussed in Hour 22, “Using Network Address Translation and Certificate Services”).

The DHCP Relay Agent is added and configured in much the same way that you added and configured the RIP routing protocol. To configure a server as a DHCP Relay Agent, follow these steps:

1. In the Routing and Remote Access snap-in, expand your server node and then expand an IP Routing node (such as IPv4).

2. Right-click the General node and select New Routing Protocol. The New Routing Protocol dialog box appears (see Figure 18.15).

Figure 18.15. Select DHCP Relay Agent in the New Routing Protocol dialog box.

3. Select DHCP Relay Agent in the routing protocol list. Then click OK. DHCP Relay Agent appears as a subnode of the IP node.

4. To add an interface (or interfaces) for the DHCP Relay Agent, right-click the DHCP Relay Agent node and select New Interface. The New Interface dialog box opens. Select the interface from the list and then click OK. The DHCP Relay Properties dialog box for the connection opens.

5. By default, the interface relays DHCP packets. You can change the hop count threshold or boot threshold for the interface. The hop count threshold is the number of Relay Agents that can relay the DHCP requests. The maximum hop count is 16. The boot threshold is the number of seconds that the Relay Agent waits before it forwards DHCP messages. In both cases, the defaults generally suffice. Click OK to continue.

6. To complete the DHCP Relay Agent configuration, right-click the DHCP Relay Agent node and select Properties (see Figure 18.16).

Figure 18.16. Configure the Relay Agent with the IP addresses of the DHCP servers.

7. Add the IP addresses of the DHCP servers to which the DHCP Relay Agent is to forward DHCP requests.

8. Click OK to close the dialog box.

The server now acts as a DHCP Relay Agent. Remember that it is necessary to configure the DHCP Relay Agent when you have multiple subnets but do not have a DHCP server on each subnet.

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