problem: MAC in via internal interface

[vladone]

Hi!
I want to permit only traffic from my network class throught internal
interface. So i put in ipfw rules this rule to see what traffic is:

$cmd add 51 count log logamount 0 all from not $local_net to any in via $lif

I study my logs and i find this:
Aug 12 09:58:14 freebsd kernel: ipfw: 51 Count UDP 0.0.0.0:68 255.255.255.255:67 in via fxp0
Aug 12 09:58:14 freebsd kernel: ipfw: 51 Count UDP 0.0.0.0:68 255.255.255.255:67 in via fxp0
Aug 12 09:58:18 freebsd kernel: ipfw: 51 Count MAC in via fxp0

When i deny this traffic, network freeze after 3-10 min. I think is
because, is blocked traffic inidcate by "MAC in via fxp0".
What kind of traffic is this? I can't block ilegal traffic to this
reason.