Today's topic is DHCP! There are three ways to get a DHCP assigned address to a client:
1) Place a DHCP server on every subnet
2) Enable BOOTP in the router(s)
3) Place a DHCP relay agent on every subnet. The relay agent picks up a client's multicast request for an IP address and then unicasts that to the DHCP server...which unicasts a response to the relay agent and then the relay agent broadcasts that IP address to the client.
If you have a DHCP server and a relay agent on the same subnet, how do you know that the server will respond first? DHCP relay agent settings can be found in RRAS, and there's a setting called "Boot threshold" which lets you tell the agent to wait several seconds to see if a DHCP server will respond.
A "split scope" is a way to create fault-tolerance for DHCP. On subnets "A" and "B", you use both a DHCP server and a DHCP relay agent. Each DHCP server can assign up to 80% of its IP addresses and the server in the other subnet holds the other 20% (the percentage is flexible). This way, if one DHCP server dies, the associated relay agent can forward requests to the other server and receive a valid address for the original subnet.
To paraphrase, a DHCPDiscover broadcast says "Hi, my MAC address is blah-blah-blah and I used to have IP address blah-blah-blah. Are there any DHCP servers available to re-assign this address to me?". It receives an IP and subnet. Then it says "Thanks, I'm also looking for a default gateway and a DNS server - do you have that info?". Here's a really good article on this topic.
Two other methods of fault-tolerance for DHCP are to cluster your DHCP servers or to use the "alternate configuration" in Windows XP.
Random note:
In a big organization, it makes sense to keep the "root domain" of your forest empty (w/ only the Administrator account active - and assigned a good password) to protect the Enterprise Admins and Schema Admins group from misuse.
