ipfw fwd and multicast mac address

[Eugene Perevyazko]

Hi

I've stumbled on a pretty strange issue in combination of ipfw fwd rules
with multicast.
 The system is 7-Stable.
 It runs ospf, that uses MC groups 224.0.0.5 and 224.0.0.6. Normally those groups use dst mac addresses of 01:00:5e:00:00:05 and 01:00:5e:00:00:06 respectively
where last 4 bytes are taken from group's IP.
 Then I needed to add some fwd rules like this:
fwd 192.168.31.14 out xmit em0
(em0 is the interface on which ospf is running)
Somehow after that MC dst mac has got 2 bytes changed:
224.0.0.5 got 01:00:5e:a8:1f:05 and 224.0.0.6 got 01:00:5e:a8:1f:06
"a8:1f" clearly is "168.31" from fwd destination.
Of course this means no ospf anymore.
I've fixed this by adding "pass dst-ip 224.0.0.0/8" before "fwd" but
it has made my evening much more lively until I figured what's happening.

The question is if this is intended consequence and if yes then why change
only two bytes instead of four (irony intended)? 

-- 
Eugene Perevyazko