On 9 October 2014 at 1:09:42 pm, Brent Marquis (brent.marquis@chorus.co.nz(mailto:brent.marquis@chorus.co.nz)) wrote:
>
> Sorry for the quick reply to myself!
>
> It actually seems like Don might not be 100% correct.
>
> I don��t have IEEE access to get the .1q standard�� But Wikipedia suggests it has been updated in 2005 for CFI to be DEI:
>
> http://en.wikipedia.org/wiki/IEEE_802.1ad
>
> Drop eligible indicator (DEI): a 1-bit field. (formerly CFI[note 1](http://en.wikipedia.org/wiki/IEEE_802.1Q#cite_note-2)[2](http://en.wikipedia.org/wiki/IEEE_802.1Q#cite_note-3)) May be used separately or in conjunction with PCP to indicate frames eligible to be dropped in the presence of congestion.[3](http://en.wikipedia.org/wiki/IEEE_802.1Q#cite_note-4)
>
> With the note suggesting �� IEEE 802.1Q-2005 clause 9.6��
>
> If it��s on Wikipedia, it must be true��. Right? J

The quote above is from the 802.1q page. If you read the page you link to, the 802.1ad page, you get:

In IEEE 802.1ad the CFI is replaced by a Drop Eligibility Indicator (DEI), increasing the functionality of the PCP field.

Key bit is ��802.1ad��, not 802.1q. Using 0x88a8 vs 0x8100/0x9100 is signalling that you��re using 802.1ad vs. stacked 802.1q, so should set this bit appropriate to the tag type.

I��m with Don on this one - the frame type bits signal how to interpret the following bits, you can��t just swap them around.

People should really just use 0x88a8 - those who aren��t, can I ask why not? Is it because you��re trying to tunnel it over a switch that doesn��t support 802.1ad or something? I��m not saying it��s wrong, I��m interested in understanding the situations in which you might do this.

Nathan Ward