How do DHCP clients pick the correct scope? 1

[pheester]

Hi, I need to deploy Windows NT DHCP in 3 subnets. First subnet merely contains servers (where DHCP will reside). The other 2 subnets relate to 2 different floors.

My question is how do clients on a certain floor know how to pick the correct scope? Do the DHCP discover packets have the default gateway added to them and then when it checks the DHCP it recognises the gateway and finds what scope has that gateway and then gives that scope's settings?

I know its possible its just I haven't seen an explanation.

PS All routers will let DHCP broadcasts through.

 

[GrnEyedLdy]

From Mark Minasi's article "Improving on DHCP". This part pertains to your question.

In case your interested in the rest of the article, here is the link.

http://www.winnetmag.com/Articles/Index.cfm?ArticleID=2662&pg=1&show=587

"If a DHCP server serves several subnets and its adjacent routers support bootp forwarding, the server must expect to receive DHCP discover broadcasts from any one of those subnets. So how does the DHCP server know which subnet the broadcast came from--how does the server know which subnet range to draw from when assigning an IP address to a client?

The answer lies in how bootp forwarding works. A bootp forwarding-enabled router will retransmit (forward) a DHCP discover broadcast. But when this router forwards the broadcast, it adds data, a note saying, "To anyone who hears this: This is a broadcast that I originally found on a different subnet, subnet x.y.z.a." Then, if a DHCP server receives a broadcast that was retransmitted over one or more routers, the server will know what subnet to direct the response back to and which scope to pull a number from for its offer".

Have fun,

Patty

 

[pheester]

Thanks for the reply.

This does sort of answer my question. However, the comment "the server will know...which scope to pull" doesnt fill my with confidence.

How does it know?

I need to deploy this shortly and will get very worried if Im stuck with this issue at 3am!!!!

 

[nat325i]

Did you ever figure this out? I need to know how to setup DHCP so that it recognizes different WAN links and serves them addresses and subnets from the correct scope...

 

[GlenJohnson]

You need a different scope for each subnet and different subnet for each floor. Check out
How to simplify Dynamic Updating
Final thought, here's a star Patty. Great work as usual.

Glen A. Johnson
If you're from Northern Illinois/Southern Wisconsin check out Tek-Tips in Chicago, Illinois Forum.

TTinChicago

http://www.johnsoncomputers.us

 

[pheester]

Thanks for everyones responses.

In the end, due to unrelated router issues we had 1 scope created that served both floors, so I never got to find out if 2 scopes on each floor would have worked

Have to wait for further expansion now......

thanks all

 

[nat325i]

Here's what I think we can do to fix my problem:

I'll need to use 2 servers because you can't have overlapping subnets in any two scopes on a server, but you can set up an overlapping subnet if you use a seperate server.

For my main server I set is as DHCP only with one scope using a subnet of 255.255.0.0

I'm going to set another DHCP server to server BOOTP addresses only with two scopes using 255.255.255.0 masks. I'll use this server to serve addresses for my remote sites. I think it'll be able to tell the difference because in the scope options it looks like you can enter an IP address for the BOOTP router. So, if a request is forwared by a given router, the server should respond with an address from the correct scope.

Can anyone confirm that this is correct.