Microsoft DNS/DHCP using DDNS - Domain Suffix issue

I have an issue with our Microsoft DNS server, we're getting the dreaded "DNS Update Failed" in the DHCP logs.

We have two forward lookup zones, blah.com and somethingelse.com - blah.com is the one I want the workstations/DHCP to dynamically update.

However, I can only get it to work if I specify blah.com as the domain suffix in the network connection properties. I can think of two possible solutions, but have no idea how to implement them or if they're possible:

1) Designate a blah.com as the "default" zone somehow on the DNS server, so all updates are sent to that zone unless the client's domain suffix is somethingelse.com

2) Use DHCP option 15, which sets the domain suffix. - We're currently doing that, but it doesn't seem to take it into account when updating DNS.

Can anyone please shed some light? Thank you.

From serverfault Samuurai

• This ended up being a problem with the clients. I had to make sure they had the correct suffix and check the box "Use this suffix when updating DNS", then DHCP happily updated DNS for me.

  joeqwerty : Normally the client will register it's primary DNS suffix (computer name FQDN suffix). If it doesn't have one (if it has a single label NetBIOS name only or is in Workgroup mode) or you want to register a different suffix, you need to do as you've done: Add the suffix to the properties of the NIC and select the option to register the connection specific suffix.
  From Samuurai