IEEE 802.1ah specification encapsulates the customer or QinQ payload in a provider header as shown in Figure: QinQ payload in provider header example.
PBB adds a regular Ethernet header where the B-DA and B-SA are the backbone destination and respectively, source MACs of the edge U-PEs. The backbone MACs (B-MACs) are used by the core N-PE devices to switch the frame through the backbone.
A special group MAC is used for the backbone destination MAC (B-DA) when handling an unknown unicast, multicast or broadcast frame. This backbone group MAC is derived from the I-service instance identifier (ISID) using the rule: a standard group OUI (01-1E-83) followed by the 24 bit ISID coded in the last three bytes of the MAC address.
The BVID (backbone VLAN ID) field is a regular DOT1Q tag and controls the size of the backbone broadcast domain. When the PBB frame is sent over a VPLS pseudowire, this field may be omitted depending on the type of pseudowire used.
The following ITAG (standard Ether-type value of 0x88E7) has the role of identifying the customer VPN to which the frame is addressed through the 24 bit ISID. Support for service QoS is provided through the priority (3 bit I-PCP) and the DEI (1 bit) fields.