After a bit more investigation, it seems that RAT running on the XP box in our AG isn't generating the appropriate (v2 membership report) IGMP packet when starting. No wonder then that the router doesn't send multicast traffic. Starting VIC does generate these IGMP packets as does RAT (same version) on a different XP box. The latter fact leads me to think that a reinstall of XP is in order.
On a related note, I wrote a quick and dirty python program to join this multicast address. And the correct IGMP packet is automagically generated by the socket.setsockopt() function call. All very strange, and happening at a much lower level than I want to explore ... Cheers, Darran.
