If_bridge and MST

[Tom Judge]

Shteryana Shopova wrote:
> On 9/4/07, Andrew Thompson <thompsa at freebsd.org> wrote:
>> On Mon, Sep 03, 2007 at 10:21:20PM +0100, Tom Judge wrote:
>>> Andrew Thompson wrote:
>>>> On Mon, Sep 03, 2007 at 02:11:59PM +0100, Tom Judge wrote:
>>>>> Hi,
>>>>>
>>>>> I was wondering if if_bridge had been taught how to speak multiple
>>>>> instance spanning tree?
>>>> Not yet. I havnt started it yet and I do not know of anyone else working
>>>> on it.
>>>>
>>> While playing with if_bridge today and a pair of Dell PowerConnect
>>> 5324's (with the recent upgrade to MSTP) I noticed that it did not seem
>>> to be possible to enable STP on if_vlan bridge members.  This would seem
>>> to be correct as transmitting STP frames tagged with VLAN_ID would seem
>>> to break the spec and fall into the realm of cisco PVST.  However should
>>> if_bridge be taught that the vlandev should be used for collection and
>>> transmission spanning tree in this scenario or should if_vlan be taught
>>> to copy untagged [R]STP frames onto the vlan interface?
>> Would this make it work the same as Cisco PVST? I havnt looked into how
>> PVST works but whatever the solution is it would need to interoperate
>> with other vendors. Obviously MST support is ideal. Most of the code is
>> already there in the form of RSTP, and MST tacks a bit more info on the
>> end. To be honest I have found 802.1Q-2003 a bit unclear in this area.
>>
>>
> 
> AFAIK, Cisco PVST is the predecessor of 802.1Q MSTP. If I remember
> correctly one of the notable differences between the two is that with
> Cisco PVST BPDUs are send for every spanning tree instance (also
> tagged?) while with 802.1Q MSTP all information is contained in the
> per instance M-records (MSTI Configuration Messages) in a single BPDU,
> and BPDUs are only sent in instance 0.
> 
> cheers,
> Shteryana

Yes this seems correct from what I have read, there is an instance for 
every VLAN, and BPDU's are transmitted tagged on each VLAN.  In MST 
instance 0 is the default instance and RSTP BPDU's are transmitted 
untagged on the interface, other instance data is attached to the 
instance 0 BPDU's as described.

Tom