MULTICAST: cannot join a group across the OpenVPN connection. Why?

I have a BRIDGEd configuration on my main LAN. The server pushes the  
"redirect gateway" option to the client. (Both server and client are  
winXP-SP2 boxes with firewall OFF)

On the main LAN there is a box MULTICASTing to the 239.255.20.1:50001  
group.
Any box on the LAN is able to join that group and properly receive the  
streamed data. If my client is physically connected to the LAN - it too is  
able to receive that stream.
But if my client is connected via OpenVPN - no data gets in.
The same time all other IP and Ethernet connectivity works as expected -  
HTTP[S], POP3|SMTP and NetBEUI - all I use for my everyday work - operate  
as they should.

What may be the cause of this misfeature?

Tony.


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