Same MAC address on two VLANs & subnets

[John Wobus]

On Feb 16, 2012, at 11:58 AM, Gerald Vogt wrote:
> Hi!
>
> Just to verify I understand correctly how the dhcp server is working.
>
> I have a DHCP 4.2.3-P2 server serving clients in various subnets, each
> subnet in a separate VLAN. The server is multi-homed through a 802.1q
> trunk serving some VLANs directly and some through DHCP relays. This
> is working fine.
>
> I have a Windows 7 computer which is also multi-homed in two VLANs
> through a 802.1q trunk (on the same NIC). It gets the IP address on
> both VLANs with DHCP.
>
> DHCP requests from Windows on either VLANs come from the same MAC
> address with the same hardware address and the same client identifier.
>
> As the DHCP server only allows one active lease for each hardware
> address/client identifier this means the windows 7 computer cannot
> have two active leases, one in each VLAN/subnet. Only the last ACKed
> DHCP request is active. Thus if the computer first gets an ip1 in
> subnet1 on vlan1 and next gets ip2 for subnet2 on vlan2, the lease for
> ip1 goes into free state again.
>
> There is no server side solution to this problem at this time. The
> only possible solution would be on the client side to make it send
> different client identifiers.
>
> Am I understanding this correctly?
>
> Or is there a configuration option which I have missed?
>
> Thanks,
>
> Gerald

As far as I am aware, the ISC server handles this fine: we
certainly see overlapping leases as folks move around.

I'm actually writing to mention that there exists network
equipment that does not handle the same mac address on
two VLANs, and depending upon what you've checked out
in that regard, its conceivable that that is causing
problems.  I also wouldn't trust a DHCP client until I'd
sniffed it.

John Wobus
Cornell