dhcp ( & cisco related ) question

Petre Bandac petre at kgb.ro
Tue Aug 8 19:36:12 UTC 2006

On Tue, 08 Aug 2006 14:18:38 -0500 Anno Domini, the honourable Ken
Roberts wrote using one of his keyboards:
> I'm going to answer what I think you're asking, rather than what you
> asked.
> What I think you're asking is, "How do I set up several VLANs to work
> with a Cisco product?"

yes, something like "how do I set up clients on several vlans gathered
in a cisco catalyst 3560 to get ips from a dhcp server"

> 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.

ok here too, with the observation that the 3560 router will act as a
gateway with the ip put on the vlan; however, this observation becomes a
nonsense if the interface which relays does not have to be "unnumbered"

> 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.

thanks for clearing things out

> 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


Petre Bandac

Network Scientist

petre at kgb.ro

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