moving devices behind dhcp relay

Simon Hobson dhcp at
Thu Mar 23 14:00:19 UTC 2006

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

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 

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.


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