VLAN & DHCP server

Glenn Satchell Glenn.Satchell at uniq.com.au
Mon Dec 17 10:42:48 UTC 2007

>From: Suhaime Raeze <suhaime at hotmail.com>
>To: <dhcp-users at isc.org>
>Subject: RE: VLAN & DHCP server
>Date: Mon, 17 Dec 2007 12:21:21 +0800
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>im trying to understand the whole process of how clients from various vlan 
receive the dhcp signal from one server..Ive read somewhere that scopes are 
needed can anybody elaborate more on that?

Ok, first thing is that as far as the dhcp server process is concerned
a VLAN and a subnet are configured the same way. The same configuration
syntax is used.

So when there is more than one subnet (or vlan) each of these requires
it's own subnet definition. For examples see the section titled
"EXAMPLES" in the dhcpd.conf man page.

For dhcp traffic to get to the server, there are two options. The
initial part of the dhcp communication uses broadcast, and typically
this is restricted to within the subnet.

1. The computer running dhcpd can have a network interface connected to
every subnet.

2. The computer running dhcpd is conected to only one subnet. Other
systems can operate as relays and forward the dhcp traffic to the dhcp
server. This functionality is built into many switches and routers. In
Cisco language it is an ip-helper.

Now in the vlan configuration, the only difference is that the multiple
interfaces are all virtual interfaces using just one hardware

So, with vlans, you can trunk all the vlans to the dhcp server, or you
can connect the dhcp server to only one vlan and use the dhcp relay in
the switch or router to forward the dhcp brodcasts.

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