Missing ID for IEEE standard 802.1ad

The yang definitions for Ethernet packet have only the (mostly) Cisco specific Q-n-Q nested tag definition.

This is a pre-standard form for provider bridging, the standard is different.

As a result, this will not work versus Linux and may or may not work versus white box silicon and other vendors which comply to the Ethernet 802.1ad standard as specified in 802.1ad.

The yang model should also include the ad tag for nested vlans - 0x88a8.