I have a problem thats bringing up something a bit basic in my mind, that I hope someone can clear up for me.
I have a trunk port on a 3560 switch that connects to a voip phone from which a PC hangs off. The phone needs to pick up a dhcp address in vlan 202, however the dhcp server is in vlan 2. I've put the following command "ip helper-address 10.11.2.200" on the router interface which has three sub-interfaces for each vlan i.e vlan 2,202 and 203.
Now when the phone issues a dhcp request the dhcp broadcast hits the router, the router then forwards the broadcast to the dhcp server (10.11.2.200), when the dhcp server receives the request it issues the address. Now my question is how does the dhcp server now what ip to give? how does it know what ip's need to be in vlan 202?
When I ran with the above scenario the phones never received a dhcp assigned address. The only way it worked was when I gave the vlan 202 interface on our 3560 switch an ip address, and then immediately the phone picked up the vlan 202 address, the vlan 202 network is 10.200.1.0 255.255.255.0.
Could someone please explain why putting an ip address on the vlan interface would suddenly allow the dhcp request to complete successfully? And how putting an ip address on the vlan helps the phone get the correct ip for vlan 202?
If you need any further info, or if I need to clarify please ask.
Thanks in advance