tcpdump -- non-local traffic not showing

[freebsd at dreamchaser.org]

Hi all,

Running 6.1

I'm having trouble seeing packets which are not going to or from the
machine on which tcpdump is running.  Is there something special I
need to do to enable this?  It's my understanding tcpdump puts the
interface in promiscuous mode, and dmesg seems to confirm this.
However I see the following behavior using "tcpdump -fntl -i ed1":

If hosts .x, .y, and .z are all on the same network,
and if tcpdump is running on host a.b.c.x
and on host a.b.c.y I do
  ping a.b.c.x

I see the icmp packets.

But if on host a.b.c.y I do
  ping a.b.c.z

I see nothing.
Does the interface drop packets with a different mac address, even
when supposedly put in promiscuous mode?

Clues?

Thanks for any insights,

Gary