About H264 and SDP negotiate between server and participaints

I find a problem:
Enable “preferH264” and disable “p2p”
I use two devices, both support VP8, but only one support h264(device A) and the other(device B) is not.
When the conference start, device A negotiate with server to use H264 and device B uses VP8. So device A send H264 to the videobridge and videobridge just relay to device B, and device B try to decode the stream using VP8(format support by it and have negotiate with server).So the decode on device B would be failed, there will be no video on screen.

I want to know why server still negotiate with device A to use H264 when device B does not support it?Does videobridge should transform the stream from H264 to VP8 before send to device B?Or just fallback to VP8 for all the participaints?

The JVB doesn’t do transcoding. It is possible to have asymmetric conferences (i.e. clients encoding with different codecs), but all your clients need to support decoding all the codecs that are used in the call (otherwise you end-up in the situation that you just described).