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DHCP and DHCP Relay Agents

By Orion IV

For the 70-291 test, a lot of focus will be on how well you understand DHCP and different scenarios where DHCP servers and DHCP Relay Agents are used.

First, you must have a thorough understanding of how to install and configure a DHCP server, as well as how to work with scopes, super-scopes, and client options.

Second, this paper will guide you through some of the more advanced topics and tricky questions you might see in one form or another at the test.

Letís examine a sample network:

We have two subnets with clients, printers, servers, DNS servers and even a RRAS server. What is missing is DHCP to ensure everyone can be assigned correct IP addresses, subnet masks, gateways, DNS servers and maybe even WINS so TCP/IP communication can begin.

So letís look at what kind of trouble or problems we can be presented with.
  1. Letís say the router is not RFC 1542 compatible so BOOTP will not be routed. How can we ensure all clients can be assigned the proper DHCP values?
    • We should place the DHCP server on one subnet, create 2 scopes (one for each subnet), and assign the client options that are specific for the scope to the individual scopes. This would be DNS (since we have 2) and the gateway. Client options for the server could be WINS if we wanted all to use the same WINS server. Then the clients would get the correct information. We might even make reservations for the printers to ensure they always get the same IP address recognized by their MAC address, and we might exclude the IP address of the File Server if it has a manually assigned IP configuration. Also, we must remember that the DHCP Server must have a static address, so we should exclude the DHCP server's IP from the scopes.
    • Since the router will not forward BOOTP or DHCP Discover broadcasts, we must ensure the clients on the other subnet can communicate with the DHCP server in some other way. We should either use another DHCP server or setup a DHCP Relay Agent. A DHCP Relay Agent will intercept the DHCP Discover broadcasts, send them to the DHCP server, get the DHCP Offer and send it back to the requesting client.
  2. Another problem could be the RRAS clients. How can we ensure they get the correct IP configuration?
    • We should assign the RRAS clients addresses from the DHCP server and ensure there is a scope for them with enough addresses. The RRAS server will borrow 10 IP addresses at a time. To ensure the RRAS clients get the client options we must install a DHCP Relay Agent on the RRAS server Ė otherwise they will only get an IP address and subnet mask.
    • If the RRAS server acts as a DHCP server itself, the RRAS clients will be given the same DNS as the RRAS Server.
  3. Let's say we want to divide Subnet B into 2 subnets. How can we ensure the clients from either of these two subnets will get the DHCP information they need?
    • Since the 2 new subnets are located on the same physical subnet side by side (a multinet) and since DHCP only can have one active scope at the time for a physical subnet, we must use a superscope.
    • First we must create 3 scopes Ė one for Subnet A and 2 for Subnet B. We then make a super-scope out of the two scopes for Subnet B.
  4. Finally we might want to incorporate redundancy in case of a DHCP Server failure. How can we prepare for this?
    • We install a DHCP server on each physical subnet (A and B); create the same 3 scopes for all subnets. In each scope we exclude all the addresses that are actively used on another physical subnet so they can be activated later, if needed, because of a failure. Then we create superscopes out of some of the scopes as needed (like in the example above).