Tcpdump showing ghost traffic

[Thomas Sevestre]

Hi all,

I'm using freebsd 8 as a router. Say I have a sis0 interface. The  
interface is up and negociated, Ethernet LEDs are fine.
Sometimes tcpdump shows traffic on the interface (tcpdump -lni sis0)  
but traffic LEDs doesn't blink and there is nothing going out of the  
router.

Do you have any idea of what the problem could be?
May be a link-layer driver bug?

Thanks,
Thomas