When the heterogeneous traffic 1 VLAN → 2 VLAN (QinQ) is interconnected or when the WAN→LAN traffic is originated there is an ambiguity, i.e. what EtherType to be used within the L2 QinQ tag. There are used several EtherType for a double VLAN:

• 0x88a8 – standard IEEE 802.1ad
• 0x9100 – QinQ, nonstandard, but still used
• 0x9200 – QinQ of the previous Cisco
• 0x8100 - IEEE 802.1Q

To resolve this ambiguity, there is the bras_qinq_type configuration option specifying the required value in the fastDPI BRAS. By default, the value of this option is equal to 0x88a8, as established by the standard.

The FastDPI BRAS uses this option value in all cases when it needs to extend the L2 headers by inserting a double VLAN. If the packet already has a double VLAN, then the EtherType type is not changed to the bras_qinq_type, the VLAN tag is modified instead.