moving devices behind dhcp relay

Sebastian Reitenbach itlistuser at
Thu Mar 23 14:08:50 UTC 2006

dhcp-users at wrote: 
> Sebastian Reitenbach wrote:
> >I want to setup some linux clients with a physical interface and a vlan
> >interface. In linux the vlan interface has the same mac address as the
> >physical one. for the vlan, a dhcp-relay is configured to route the dhcp
> >request to the server. is the dhcpd able give different ip addresses to the
> >physical interface and the vlan interface despite they have the same mac
> >address?
> So you want to use the real interface (eg eth0) as well as a separate 
> VLan interface (eg eth0.123) at the same time ?
>  From my understanding of how the system works, I'm not sure you can 
> do it at all. How, for example, does the switch know the difference 
> between a tagged and untagged packet ? All the switches I've worked 
> with can have a port either tagged or untagged, not both at the same 
> time.
yes, the switch can do that, it makes all untagged packets in vlan1 and all 
tagged packets stay in their tagged vlan.

> If you do get around that, then DHCP can deliver two addresses if 
> asked for them. You will need to get the dhcp client to request an 
> address for each interface, and assuming that the logical interfaces 
> are in different subnets, I think you'll find that having the same 
> MAC address won't matter.
ok, sounds great, as it is exactly as I want it.

thanks a lot

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