dhcp ( & cisco related ) question

Ken Roberts ken at hoverclub.net
Tue Aug 8 19:18:38 UTC 2006

I'm going to answer what I think you're asking, rather than what you
What I think you're asking is, "How do I set up several VLANs to work
with a Cisco product?"

First, I did not use 802.1q on the Linux box.  I put that on a subnet
with an IP address, just like anything else.  There's no real reason to
put trunking onto the DHCP server.

The pertinent configuration for the Cisco gear (assuming a 3550 layer 3
switch) is:
ip dhcp relay information option
no ip dhcp relay information check

and then, for each VLAN:
ip helper-address
ip helper-address

For your dhcp configuration, create your subnets the way you would
think.  The router will send along enough information so that dhcpd can
figure out which subnet you're on.

Sorry if this doesn't quite go where you wanted to go.  Not that I'm an
expert on this, but it seems to me that extending a trunk to a host
without good cause is a security hole, and the gear is all designed to
make your life easier anyway so it's unnecessary in this case.

On Tue, 2006-08-08 at 13:51, Petre Bandac wrote:

> hallo
> I have a network topology which requires me to serve dhcp ip addresses
> on several 802.1q vlans 
> first question: 
> assuming I bring up several dot1q interfaces on the linux box, do I
> have to put an ip address from every subnet dhcp serves ? (dhcp server
> is not the same box as the gateway, is it possible to make it on a
> "broadcast" manner ?)
> the main reason of this question (if I put it correctly) is not to lose
> so many ip addresses when subnetting - two for net address and
> broadcast and two more - one on the router as gw and one for the dhcp
> server
> second question:
> the dhcp relay feature of the cisco routers enables it as a relay host
> for dhcp messages from another server ?
> at least that is what I understood from
> http://www.cisco.com/en/US/products/sw/iosswrel/ps1834/products_feature_guide09186a00800800ed.html
> thanks for your time,
> petre

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