Hello - I just want to get some general guidance on this. We're at capacity on our 192.168.1.x /24 subnet, and are adding a 2nd subnet to our DHCP server for 192.168.2.x / 24. We've had Cisco add another VLAN to our network so that both VLANs 1& 2 can talk to each other. What I need clarity on is when adding the 2nd subnet to the DHCP server, how do I control what clients take addresses from 1 subnet as opposed to the other? Do I just statically address machines to control that? Or do I NEED to worry about that at all? Will DHCP automatically hand out addresses from the 2nd one when the 1st one gets down to 0 IPs left?
Thanks for your help.
1) Broadcast DHCP requests. - Since it is a broadcast, the DHCP server knows that it must be from the network that the DHCP server is on because broadcasts are local to the network. Therefore it gets a DHCP request as a broadcast on the interface on the 192.168.1.x network. It then responds with an address out of that pool.
2) DHCP relay requests - These are DHCP unicast packets that are sent from a router in order to relay a DHCP request from another network segment. When the DHCP server receives these packets, the relay packet has the network from where the request is coming in the packet. The DHCP server sees this and then knows which scope to send an IP address from.
Now to make it work on the DHCP server you just need to build your scope and activate it.
So on the layer 3 device that is handling the default gateway function for the new network is where you will need to configure the dhcp relay.
So for instance it might look like the following:
If on layer 3 switch:
interface vlan 2
ip address 192.168.2.1 255.255.255.0
ip helper-address 192.168.1.10
The address defined as the ip helper-address is the address of your dhcp server.
That is pretty much all you need to do.