Specifying interface for iperf

[<cn...@ny...>]

Running iperf 2.0 on a Linux system with 2 interfaces, I do:

   iperf -s -u -B 224.0.55.55 -i 1 &

and I see:

   Server listening on UDP port 5001
   Binding to local address 224.0.12.34
   Joining multicast group  224.0.12.34
   Receiving 1470 byte datagrams
   UDP buffer size: 64.0 KByte (default)

but I _don't_ see any join message on my network of interest.  I'm guessing it went out the other interface.  Can I specify the interface to use?  I don't see any option for that in the docs.

Thanks.

                              Chris