I am doing some work at a client that has 25 offices. They have a central DHCP server at the main office. (We won't go into how that is a bad idea right now.)
Each of the 25 offices have the same VLANS, vlan 110 for data, vlan 120 for voice. Each office has their own 10.x.x.x/16 IP space.
On each of the VLANs there is a helper IP address for the DHCP server pointing back to the central DHCP server.
My question is this --- how does the DHCP server know when it hears a DHCP for VLAN110 in office A to give an IP from the proper scope and not from the scope VLAN 101 of office B?
I suspect the DHCP server somehow determines this from the source IP of the router/switch that has the help IP configured on it local VLAN.