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

Stephen Hemminger shemminger at osdl.org
Wed Dec 13 10:24:25 PST 2006

On Fri, 08 Dec 2006 18:24:07 -0500
Cameron Schaus <cam at schaus.ca> wrote:

> 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
> _______________________________________________
> Bridge mailing list
> Bridge at lists.osdl.org
> https://lists.osdl.org/mailman/listinfo/bridge

The change was intentional because the bridge follows the 802 spec
and doesn't pass link local multicast frames. 

If you are running STP on the network, you need to run STP on
the bridge.

More information about the Bridge mailing list