Deny DHCP lease to particular classid

I’m having some troubles implementing classids in my current environment.

The situation I have is as follows: DomainA and DomainB reside within the same network, each has their own DHCP/DNS/AD etc. There is a one-way trust between Domain A+B.

I have set a user class of ‘domainA’ within the DHCP scope on domainA (duh), set the classid on the computers within domainA and left domainB without a classid.

For reasons outside this discussion, I require two DHCP servers.

The problems I am having are as follows:
1. PC within domainB are able to receive a lease from domainA
2. PCs in domainA (with a classid set) are receiving leases from domainB
3. PCs in domainA aren’t receiving a lease from domainA

Any thoughts appreciated.

-TimmyQ