[Bridge] BPDU's not passing through bridge when STP is disabled

[Cameron Schaus <cam@schaus.ca>]

I have noticed a change in the linux bridge implementation between 
2.1.15 and 2.1.17.  Specifically, I do not think BPDU's (generated from 
another bridge) are passed across the bridge when STP is disabled.  I 
think this relates to the LLC handling of BPDU's directly invoking 
br_bpdu_rcv.

In 2.6.15, the br_handle_frame function would pass a BPDU to the 
br_handle_frame_finish function which would transmit it across the 
bridge.  But in 2.6.17, the br_bpdu_rcv function is invoked directly by 
the LLC layer, which does nothing if stp is disabled.

Was this change intentional?  The reason I ask is that I can introduce a 
loop into the network by doing the following:

Two machines, both with eth0 and eth1 bridged and STP is enabled:

        ----- A -----
-----+                +------
        ----- B -----


When I disable STP on machine A, it reverts to a standard bridge, with 
both eth0 and eth1 in the forwarding state.  Bridge B has STP enabled, 
and is sending out BPDU's.  Bridge B should disable one of its ports to 
prevent a loop in the network, but it doesn't, because bridge A does not 
pass the BPDU's across, so bridge B cannot detect the loop.  With the 
2.6.15 kernel, this loop was prevented.

Can br_handle_frame_finish be called from br_stp_rcv in the event that 
stp is not enabled, or would doing so cause problems?

Cam