[2008 server] Multicast frames on a specified interface

Hi,

I got a problem sending big UDP frames (size > MTU) if I specify the sending
interface (IP_MULTICAST_IF).

Using SDK 6's mcastws1 sample, and wireshark filtering on host
229.230.231.234.
mcastws1.exe -s -n 5 -m 229.230.231.234 -z 1500 -u -t 1
No multicast interface specified : frames are sent.

172.16.136.1 is a host IP address :
mcastws1.exe -s -n 5 -m 229.230.231.234 -i 172.16.136.1 -z 1500 -u -t 1
Wireshark only see IGMP management frames. Remote listener don't receive
frames, too.

mcastws1.exe -s -n 5 -m 229.230.231.234 -i 172.16.136.1 -z 1400 -u -t 1
Frames size < MTU : sent.

This comportement is the same on Vista. Using a upper version of winsock
(2.0, 2.2) does not change anything.

What's the problem ??