DHCP on VLAN

[Simon Hobson]

Kenneth Ling wrote:

>so did the dhcp server need to have all vlan IP ?

See below

>eg.
>assume my vlan ip are
>192.168.1.0/24
>192.168.2.0/24
>192.168.3.0.24
>and my dhcp server is 192.168.100.100/24
>do i need to configure 192.168.1.1/24, 192.168.2.1/24 in the dhcp server ,
>and the cisco

Simple answer - we don't and CANNOT know the answer to that since we 
don't know what your network is set up like ! However, I'll have a 
guess at two likely configs and you can correct me if I'm wrong ...


config 1
I'm assuming that you have a port on the switch configured as a trunk 
port, and this is connected to your server. I'll further assume that 
the server is also doing routing between subnets.

This requires that you configure VLAN tagging on the ethernet port 
connected to the switch trunk port AND that you configure a virtual 
interface for each VLAN that you wish to use. Note that I do NOT mean 
configure multiple IP addresses on one port (aka aliases) such as 
eth0:1 as 192.168.1.1, I mean that you must configure a VLAN 
interface (eg vlan1 at eth0) for each vlan.

You must also configure a subnet declaration for each subnet in your 
dhcp config. However, since each subnet is directly attached to the 
dhcp server, you do NOT configure a relay agent in the switch.

This setup may be considered as analogous to having multiple ethernet 
ports in the server, each connected to a separate switch. All the 
VLAN stuff does is allow you to condense all the separate ports and 
switches into one with some packet tags and software taking care of 
the segregation instead of physically segregating the networks.


config 2
For this one I'll assume you have level 3 routing ability in the 
switch, and the server is only directly connected to one VLAN. In 
this mode, the routing between networks is done by the router in the 
switch and you must configure a relay agent on every network except 
the one to which the server is directly connected. The dhcp server 
will see all but one of the networks as remote networks,  but that is 
handled automagically. You will have one ethernet port with one IP 
address connected to a non-trunk port on the switch.

You still need the same dhcp configuration with subnet declarations 
for all the subnets.