[Bug 264014] QinQ not working with a lot of switches

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=264014

--- Comment #1 from Zhenlei Huang <zlei@FreeBSD.org> ---
The IEEE 802.1AD published at May 26,2006 [1]. That is about 17 years ago at
the time writing.

> There are a lot of switches on the market with QinQ support, but not all support
> QinQ according to IEEE 802.1AD. This is because of historical reasons. Before IEEE 
> finished 802.1AD, vendores implemented pre-802.1AD standards, often called 802.1QinQ
> or something like that. 

To be honest, it does not require much effort to support them in FreeBSD net
stack. But I am *personally* not willing to do so, as these devices are
manufactured at ancient times. Probably they are EOL and out of support event
by the vendors.

> Today, you can still buy modern switches with legacy/proprietary QinQ
> implementation. Those use EhterTypes like 0x9100 for example. Some but not much 
> switches allow to modify the EtherType to  0x88A8 according to 802.1AD or set the 
> EtherType of QinQ to custom values (for example Dell OS6 allows both).

So the default EtherType should be 0x88A8 (802.1AD). 0x9100 should be chosen
only for interoperability with ancient devices.

1. https://www.ieee802.org/1/pages/802.1ad.html

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