Similar to PW transport, operators want to allow their customers to preserve all eight Ethernet CoS markings (three dot1p bits) and the discard eligibility indication (DE bit) while transiting through a PBB backbone.
This means any customer CoS marking on the packets inbound to the ingress SAP must be preserved when going out on the egress SAP at the remote PBB PE even if the customer VLAN tag is used for SAP identification at the ingress.
A solution to the above requirements is depicted in Figure: PCP, DE bits transparency in PBB .
The PBB BVPLS is represented by the blue pipe in the middle with its associated CoS represented through both the service (I-tag) and tunnel CoS (BVID dot1p+DE or PW EXP bits).
The customer CoS is contained in the orange dot1q VLAN tags managed in the customer domains. There may be one (CVID) or two (CVID, SVID) tags used to provide service classification at the SAP. IVPLS or PBB Epipe instances (orange triangles) are used to provide a Carrier-of-Carrier service.
As the VLAN tags are stripped at the ingress SAP and added back at the egress SAP, the PBB implementation must provide a way to maintain the customer QoS marking. This is done using a force-qtag-forwarding configuration on a per IVPLS/Epipe basis under the node specifying the up link to the related BVPLS. When force-qtag-forwarding is enabled, a new VLAN tag is added right after the C-MAC addresses using the configured qtag. The dot1p, DE bits from the specified outer/inner customer qtag are copied in the newly added tag.
If the force-qtag-forwarding is enabled in one IVPLS/PBB Epipe instance, it is enabled in all of the related instances.
At the remote PBB PE/BEB on the egress SAPs or SDPs, the first qtag after the C-MAC addresses is removed and its dot1p, DE bits are copied in the newly added customer qtags.