DHCP when using default VLANs

Question

I have a question about Windows DHCP scopes.

Site A is local and has scope 10.0.0.1/24 and hosts the Windows DHCP Server on 10.0.0.5/24. No devices are on the default VLAN.

Site B is remote. Devices are on the default VLAN. The DHCP Server (router) is being retired, and we need to use IP helper and forward DHCP requests from 192.168.4.0/24 to Site A DHCP server(10.0.0.5), and have the server provide the IP address going forward for the 192.168.4.0/24 scope that is remote.

There will be a second DHCP scope on Site A DHCP server for the remote address pool.

Without defined VLANs in site B, will devices in site A consume IP addresses from the second scope created for 192.168.4.0/24?

Thanks

Answer 1

Yes. You need a DHCP relay on site B with an interface in 192.168.4.0/24, routing DHCP requests to your DHCP server. On the DHCP server you need a scope for 192.168.4.0/24 (and obviously, support for relayed requests).

Whether you use VLANs, tagged or untagged, doesn't really matter (of course, VLAN configs must match on both ends of each link). As long as your DHCP server accepts relayed requests, there's a relay in each VLAN, and proper routing in between, DHCP relaying is going to work.

Answer 2

No, because DHCP isn't able to issue IPs without any interface in that network. DHCP is layer3, so a long as there aren't any interface with 192.168.4.0 addresses, it is technically impossible to grab adresses from that range.

The routing protocol "dhcp relay" (usually on IP helpers) has to change the broadcast adress of the client(s) to a (routable) unicast to overcome this for remote networks.