Same MAC address on two VLANs & subnets

Gerald Vogt vogt at spamcop.net
Fri Feb 24 08:51:41 UTC 2012


Doesn't anybody here know whether a single client-id can have two
active leases in two different subnets?

Did noone ever used VLANs on a Windows box to connect into two
different subnets with DHCP?

Thx, Gerald

On Fri, Feb 17, 2012 at 7:44 PM, Gerald Vogt <vogt at spamcop.net> wrote:
> On 17.02.12 18:02, Simon Hobson wrote:
>> Can you post your config and log entries for this host ?
>
> The complete config is quite long. But the problem comes down to a
> simple config like this:
>
> # VLAN 10 subnet
> subnet 10.10.10.0 netmask 255.255.255.0 {
>  option routers 10.10.10.254;
>
>  range 10.10.10.1 10.10.10.100;
> }
>
> # VLAN 20 subnet
> subnet 10.20.20.0 netmask 255.255.255.0 {
>  option routers 10.20.20.254;
>
>  range 10.20.20.1 10.20.20.100;
> }
>
> A client with a VLAN/802.1q capable NIC connects into both VLANs and
> tries to get an IP address with DHCP from both subnets.
>
> The standard logs won't show much. I have a debug log with more
> information but no access to it at the moment. But what I have learned
> from the logs and the source code is this:
>
> The while loop in dhcp.c:3409 goes through all leases for the given
> client identifier. If it finds a lease with binding state active but in
> a different subnet (line 3435) it jumps to line 3451 where the lease is
> released in line 3456.
>
> For example, if the client first gets the IP address 10.10.10.5 on VLAN
> 10 and then does a DHCPDISCOVER on VLAN 20 I see this in the debug log:
>
> trying next lease matching client id: 10.10.10.5
> wrong network segment: 10.10.10.5
>
> followed by the message that lease 10.10.10.5 goes from active to free.
>
> Looking at that while loop I am pretty sure that any given client
> identifier can only have a single active lease at any time...
>
> Gerald
>


More information about the dhcp-users mailing list