DHCP

Assuming you will have equal sized scopes (500 on each server or something similar) (and without the scopes overlapping), the only issue will be if you have more clients asking for leases than a single scope can handle. Since DHCP requests are broadcasts, the first DHCP server to hear it will answer. It should not make any difference if a computer accepts an IP address from one server one time and then from another the next.

You will lose redundancy, and if you use the same scope for two different IP addresses there will be errors. Make sure that you have a different pool for the DHCP set and start in different ranges. Do not have the same pools or you will get errors, otherwise you won't have any problems.

Lets call your PDC Server A, your BDC Server B as it doesn't matter regarding DHCP anyway.
The total amount of IP addresses to be leased at any one time is 1000, I would hope that you have more than one BDC as 1000 pcs being leased an IP using DORAwill I would say slow down the servers at logon, until everyone has logged on the servers will be busy responding to the UDP broadcasts.
If having only two Servers I would do the following, this example takes into account one of your servers DHCP service failing for examle:
Both Servers A and B will have two scopes (each 500) with no overlapping IP addresses, Server As first scope will be active, the second scope will not be. Server Bs first scope is not active while it's second scope is.
So I would imagine this is the way your dealing with it, 2 scopes on each server with only one scope active on each server.
In the case of Server A failing all you need to do is activate the first scope on server B. As you know DORA broadcasts are 50% and 75% of the leased time so at when 50% of the lease has gone your clients will then attempt to renew the lease of their IP with the server that issued it, yes it will attempt contact with the same server, at this point it will fail to renew your clients IPs, at 75% of lease time expired your clients will this time broadcast for either a new IP or renewal, if Server A is still down then your clients will get a new IP from Server B who has the whole scope activated. When Server A comes backonline deactive one Scope on Server B and activate one on Server A, the sequence will repeat until your server are serving half your clients each.

Hope this is of help

A couple of things you did not mention...
1. How many pc's do you have that leases IP's?
2. Are all these PC's at one location?

Obviously if you have more than 500 pc's both servers will lease addresses out.You could split the addresses between2 servers for redundancy if you want to: meaning that if your PDC goes down you can activate the BDC when it is promoted to PDC.
If that is not the idea and you want both to work at the same time and lease out IP's then you have to make very sure that the 2 scopes does not overlap. If you have more than 500 clients it will be a problem if you do not have the second scope on the remaining server. The client will send a request for renewal to all servers on the net if it does not get an answer from the original server it leased from. If the same scope is activated on the BDC it will then renew the lease.
Just make sure that when you bring the PDC back online that you then de-activate the "PDC" scope from the BDC when you do the whole "promoting and demoting" thing

Hope this helps a bit

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