[nsp] Auxillary VLAN and DHCP

[Andrew Melton]

I have a bunch of 7940 IP phones connected to a 4506.  The aux vlan is
set and is configured.  We have two DHCP scopes, one for the default
vlan, one for the aux vlan, both on the same machine.  The server
itself is on the default vlan.  There is a layer 3 subinterface on a
router uplink which routes between the two vlans.  That subinterface
has an ip helper command pointing to the DHCP server on the default
vlan.

Problem is, it seems that as soon as the phone is plugged in, I see a
broadcast request for a DHCP server.  A split second later, there is
another broadcast request for DHCP, originating from the aux vlan IP
address.

What happens, is that the two requests come from the same MAC address.
So, the first one is answered by the DHCP server, but rejected either
by the phone or the IP subinterface routing aux vlan traffic.  Then,
since there is already a lease assigned in the DHCP server, the
second, legit request is denied.

The only way we can get the proper IP is for the default vlan scope to
be disabled when the phone is plugged in.  This shouldn't be the case.
Any thoughts?