[ovs-dev] [PATCH] rstp: add ability to receive VLAN-tagged BPDUs

Matthias May matthias.may at neratec.com
Mon Feb 18 11:07:54 UTC 2019


On 15/02/2019 01:28, Ben Pfaff wrote:
> On Fri, Feb 15, 2019 at 12:27:11AM +0100, Matthias May wrote:
>> On 14/02/2019 20:17, Ben Pfaff wrote:
>>> On Thu, Feb 14, 2019 at 10:58:48AM +0100, Matthias May via dev wrote:
>>>> There are switches which allow to transmit their BPDUs VLAN-tagged.
>>>> With this change OVS is able to receive VLAN-tagged BPDUs, but still
>>>> transmits its own BPDUs untagged.
>>>> This was tested against Westermo RFI-207-F4G-T3G.
>>>>
>>>> Signed-off-by: Matthias May <matthias.may at neratec.com>
>>>
>>> Thanks for the patch.
>>>
>>> To me, it seems really odd to treat packets with and without an
>>> arbitrary VLAN header the same way.  I could see it if the VLAN header
>>> had VID 0 or 1 or some other specified value, but it seems unusual to
>>> ignore it entirely.  Is this standardized or a de facto standard of some
>>> kind?
>>>
>>
>> I totally agree.
>> To me a VLAN header has nothing lost on a BPDU of a (R)STP frame, simply
>> because (R)STP is not per VLAN.
>>
>> However the fact is that there are switches which are transmitting
>> frames on a VLAN.
>>
>> With this change we simply ignore the VLAN header if is present. The
>> meaning of the BPDU doesn't cheange. The provided information still is
>> not per VLAN and applies to all ports the same.
>>
>> This patch does not add the ability to transmit VLAN tagged BPDUs for
>> the same reasoning above: RSTP/STP is not supposed to be per VLAN.
>> I was thinking about adding to the patch that one can specify a VLAN via
>> config and only BPDUs with the configured VLAN are accepted. I guess
>> this is what you propose: only accept vlan tagged BPDUs on a specified VLAN.
>> Having such a config-parameter would also enable to transmit the BPDUs
>> VLAN tagged. But I'm still of the opinion that this only suggests that
>> one could have an (R)STP tree per VLAN.
> 
> I see we are basically in agreement, but I'd like more information, if
> you have it.
> 
> Do the switches that transmit RSTP on a VLAN transmit it on a particular
> VLAN like 0 or 1?  (Maybe they are transmitting them as priority-tagged
> frames for some reason?)  If so, then it would be possible to accept
> just that VLAN.
> 

Hi Ben
The switch I tested against has config options to specify with which VLAN the BPDUs should be transmitted.
>From the tests I did, for this switch it doesn't matter if the received BPDUs are tagged or not. The VLAN header is
simply ignored.
I have an open question with the vendor regarding this.

I did some digging on the net how other switches handle this. E.g at [1].
>From what I get, switches which allow tagged BPDUs simply ignore the header.

I started reading the 802.1d-2004 and 802.1q-2018, but so far haven't found anything which specifies how this situation
should be handled.

BR
Matthias


[1]
https://community.extremenetworks.com/extremeswitching-eos-230140/what-happens-if-an-s-series-receives-tagged-rstp-bpdus-7765211


More information about the dev mailing list