Segmenting a network with 3750

[Bucky101]

I have 2 stacks of switches, voice and data which are supposed to be segmented by the Cisco 3750. The voice stack has its own DHCP server and so does the data stack.
I have created a layer 3 interface on the 3750 by going

interface GigabitEthernet1/0/21
no switchport
ip address 128.68.100.1 255.255.255.0

And plugged interface 21 into the voice stack. The problem is, I am getting both DHCP accross both these switch stacks which means the network is flat and is not being segmented properly by the 3750. What have I done wrong?

 

[VinceWhirlwind]

Your first problem is a problem of basic architecture: You have separate physical networks when what you should have are separate virtual networks.

From there, you expect the two networks to be "segmented" by a Cisco 3750, but it isn't clear how you think this is being done.
If the two switch stacks are not patched into each other, then you have two separate physical networks.
If you want to patch them together, you need to ask yourself what the purpose of the connection is going to be.
You talk about creating a layer3 interface on one of the switch stacks, but you don't explain what it's patched to on the other switch stack. What you've done doesn't make a whole lot of sense without the context of what you're trying to achieve.
Creating layer3 interfaces also begs the question of how you're going to manage these switches, what subnets are these interfaces in, why you don't make a layer2 connection between the switch stacks and segment your networks using VLANs as is the norm, and a bunch of other questions....