DHCP Experts (Windows 2000 Server)

[Sianath]

This is a possible physical topology, but is it possible to serve this network with a single DHCP Server.

Consider the following:

DHCP Server connected to Router #1
Router #2 and Router #3 connected to Router #1

Subnet A is separated from the DHCP server by Router #2
Subnet B is separated from the DHCP server by Router #1
Subnet C is separated from the DHCP server by Router #3

DHCP Server:
Scope A: 192.168.1.1 - 192.168.1.254 /24
Scope B: 192.168.2.1 - 192.168.2.254 /24
Scope C: 192.168.3.1 - 192.168.3.254 /24



Now, my question is this.

Is it possible to configure the DHCP Server to lease out IP addresses from a specific scope to a specific subnet. (i.e. When a computer from Subnet A requests an IP address, the DHCP leases from Scope A, etc)

Assume either of the possible scenarios to make this work (DHCP Relay Agent on each subnet, or routers that forward BootP)

Is it possible to configure? Or are you stuck with having to use a separate DHCP Server for each subnet you want to serve.

And yes, I'm aware of the information that is passed from client to server when requesting IP's and the process involved, so please don't respond immediately with "There's no way for the DHCP Server to know where the client is located", I can think of three possible ways it can figure that out... just not a way to set an option to use that information.

Looking for real answers here

 

[ScottMac]

As long as your clients can present a "Scope" parameter to the server, it should work with the following caveat:

You need to configure "Ip helper" (or whatever similar process your router offers). DHCP requests are a broadcast, the routers will stop broadcasts. "IP Helper" on the ingress port of the router still kills the broadcast, but "converts" the request to a unicast directed at the DHCP server.

Good Luck

Scott

 

[Nothinman]

I havn't personally done it (and I'm too lazy to open my DHCP book right now) but what would be the point of DHCP relay agents if the DHCP server couldn't give the clients it's forwarding for IPs?

 

[bozo1]

I've had hundreds of subnets being served by one DHCP server before. Just create the scopes with a proper ip range for that subnet, have DHCP assign the proper default gateway for that subnet and make sure your routers are passing dhcp/bootp requests.

 

[Sianath]

bozo1 -> How does it know to lease an IP from a specific scope for that subnet and not just whatever is available, does it use the IP of the router that forwarded the address and compare to the available scopes?

ScottMac -> what you are thinking is setting the router to forward BootP (which is UDP ports 67 and 68) which is one of the possible scenarios I outlined My question deals with how the DHCP Server will know to give a specific IP from a specific scope for a specific subnet, considering the only information a client passes to the Server when requesting an IP address is a default IP of 0.0.0.0 and it's MAC address.

Nothingman -> The point of a DHCP Relay Agent is to get the IP request past the router. Routers don't forward broadcasts automatically, so the Relay Agent listens for those broadcasts, and forwards them to a specific IP you specify when setting up the Agent.

 

[bozo1]

<< does it use the IP of the router that forwarded the address and compare to the available scopes >>


That's how it works. And if the server receives a request from a subnet that is not configured for DHCP, it just ignores it. If the router is configured to pass those UDP ports, that's all you need. If you are not allowing UDP broadcasts, then you need to set an ip-helper address on the router that defines the address of your DHCP box.

 

[Sianath]

Awesome! Thank you!

I suspected it was possible this way, but I wasn't sure if it was an option you had to set from the scope itself. I was digging through them all and couldn't find anything, so I figured I'd just ask.

 

[bozo1]

For redundancy, I always have 2 DHCP servers on my network each giving out a different range of IP addresses for each scope in case one dies:

Server A - 192.168.1.10 - 192.168.1.100
Server B - 192.168.1.101 - 192.168.1.200

or similar. Both DHCP servers will reply to the request and the workstation will take the first reply. Since the 2nd reply wasn't acknowledged, that server just puts that IP address back into the pool.

 

[Nothinman]

Nothingman -> The point of a DHCP Relay Agent is to get the IP request past the router. Routers don't forward broadcasts automatically, so the Relay Agent listens for those broadcasts, and forwards them to a specific IP you specify when setting up the Agent.

I realize that, my question was supposed to be rhetorical =)

 

[Saltin]

Bozo, do you usually use a 50/50 scope split like that?
I usually go 80/20. Maybe that's the Microsoft man in me .......

 

[bozo1]

Yea, got into the habit of do 50/50 years ago when we had some subnets with a 300-400 machines on them and had a DHCP server on different backbones for redundancy there as well. (Token Ring, 255.255.252 subnet mask, don't ask) I needed to be able to supply enough addresses to most, if not all machines if one of my servers or backbones was down.