one DHCPD server multi subnets

[tangaish.en]

hi,everyone!

I'm new to the list, I read isc dhcpd document and googled a lot about
multi subnet dhcpd case, but I can't finure out, so I ask here.

I have a network with 5 subnet (VLAN 1 to 5),ip address is 172.16.1.0/24
to 172.16.5.0/24, 5 subnet divided by a 3-layer switch, all subnet
gateway(172.16.X.1) is 3-layer switch vlan interface, from
vlan-interface1 to vlan-interface5, the 3-layer switch do ip routing.
the dhcpd server is in 172.16.1.0/24 subnet, ip address is
172.16.1.2/24, default gateway 172.16.1.1 , this subnet self not require
dhcp service.

all desktop/laptop/printer is in subnet 172.16.2.0/24 to 172.16.5.0/24,
divided by department, one vlan meet one department, these clients
require dhcp service.


my plan is:

add "ip helper-address 172.16.1.2" configure in switch vlan interface
from 2 to 5, so the client DHCP request will be forwarded to ISC DHCPD
server, but my question is how isc dhcpd classify the DHCP request is
coming from?

I read dhcpd conf and found class and sub-class, it work when I know the
client MAC address and the client will get the correct ip address
belongs to the vlan they are, but if a new laptop plug in the network,
dhcpd don't know how to assign ip address to the new laptop, I don't
think fix MAC address ip assignment is a good idea, for it's a horrible
thing to manage MAC addresss in a big network.

I don't know much detail of the dhcp protocol, but I think the option in
dhcpd.conf "option dhcp-client-identifier, hardware" can rewrite the
keywork "hardware" to something other, such as "source subnet" or
"source relay address" etc? how can I find more dhcp-client-identifier
parameter?

thanks.