Same MAC address on two VLANs & subnets

[Chris Buxton]

On Feb 24, 2012, at 7:18 AM, Gerald Vogt wrote:

> On Fri, Feb 24, 2012 at 3:38 PM, John Wobus <jw354 at cornell.edu> wrote:
>> On Feb 16, 2012, at 11:58 AM, Gerald Vogt wrote:
>> 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.
> 
> The network handles the same MAC address on multiple VLANs properly.
> All DHCP messages are sent to the correct VLANs and are received on
> the correct VLAN on the server side.
> 
> It's definitively the server: the very moment it receives a
> DHCPDISCOVER on the other VLAN/subnet it releases the lease in the
> other subnet. I can see it in the dhcpd.leases file: it first writes
> an "binding state released" entry for the already active lease
> 10.10.10.5, immediately followed by the new active lease for the
> second subnet 10.20.20.8. The client definitively does not release the
> 10.10.10.5 address not does the server receives any other messages but
> the DHCPDISCOVER and DHCPREQUEST for the second address on the second
> VLAN.
> 
> I can only find that it's the server which releases the first lease
> because the client uses the same client identifier to obtain the lease
> on the second subnet. I am puzzled if others are able to discover two
> leases in two subnets using the same client identifier. I can't see
> that at least for 4.2.3-P2.

If you don't have one-lease-per-client enabled, then you may have found a bug in dhcpd. It certainly works for a given MAC address, so the question is, does it work for a given client ID?

Regards,
Chris Buxton
BlueCat Networks