Hi trd87t,
I recommend you to capture the DHCP Discover packet from Aruba controller and look at "Relay agent IP address".
In my case with Catalyst 2960 switch, if the port for Aruba controller is Fa0/10 and monitoring port is Fa0/48,
conf t
(config)monitor session 1 source interface fa0/10
(config)monitor session 1 destination interface fa0/48
(config)end
Then I connected a laptop with wireshark installed.
Select Capture - Option and choose interface. Click on "Capture Filter" and choose "TCP or UDP port 80 (HTTP) and chagne filer string as "port 68" Click OK.
On Capture - Option screen click Start to capture the trace.
You only see DHCP (bootp) protocol in the screen.
If you see DHCP Discover packet, find "Bootp flags: 0x0000 (unicast)
Client IP address 0.0.0.0 (0.0.0.0)
Your (client) IP address 0.0.0.0 (0.0.0.0)
Next sever IP address 0.0.0.0 (0.0.0.0)
Relay agent IP address 0.0.0.0 (0.0.0.0)
In order the central DHCP server to handle DHCP scope correctly, Aruba should set Relay agent IP addres as VLAN interface's IP address. If your VLAN100's interface IP is 172.21.100.1, this Relay agent IP address should be set as 172.21.100.1, so that central DHCP server can recognize which DHCP scope to respond. If the central DHCP server has a scope 172.21.100.x, the central DHCP server should send DHCP Offer with 172.21.100.x.