Figure: DE aware PBB topology shows an example of a PBB topology where the DE feature can be used. The processing requirements highlighted in the 802.1ad use case apply to the 802.1ah BVID, format and etype, these being identical with the 802.1ad S-TAG. In addition, the DE bit from the 802.1ah ITAG header may need to be processed following the same rules as for the related field in the BVID/S-TAG; for example, the DE bit from the BVID header represents the QoS associated with the ‟Ethernet Tunnel” while the DE bit from the ITAG represents the service QoS.
In this example, the BVID is not used for a part of the network, leaving the I-DEI bit from the ITAG as the only option for a dedicated DE field. If both are included, the QoS information from the BVID is to be used.