The introduction of Provider Backbone Bridges (PBB) is a significant step in the evolution of Carrier Ethernet. It is the step from Provider Bridges( PB) to Provider Backbone Bridges. PBB technology, which includes a 16- byte header extension, significantly expands the maximum number of customer-specific VLANs, which is limited to 4,096 for Provider Bridges (PB). As a result, expandability in large city networks is no longer limited.
Provider Backbone Bridges (PBB) are defined in IEEE specification802.1ah, they extend the scaling capabilities of Provider Bridges (PB) and also provide a migration path to hierarchical Ethernet infrastructures.
The PBB technique adds another MAC header, the PBB MAC header, to the original Ethernet frame. This technique is also referred to as MAC-in-MAC technology. The PBB MAC header is used by the core switches in the backbone of the core network, which only switch frames with a B header (backbone). The original MAC header, on the other hand, is used in the client infrastructure.
In the case of provider backbone bridges, the original MAC header is supplemented by the backbone header. The B header consists of two 6-byte bridge addresses for the destination address and the source address (B- DA and B- SA). The Backbone Destination Address (B-DA) is the destination address from the edgeswitch of the PBB network, the Backbone Source Address (B-SA) stands for the MAC address of the sending edge bridge. The optional Backbone VLAN Identifier (B- VID) data field represents the VLAN Identifier (VID) and ensures that the frame takes the correct path. The 4-byte Instance Tag (I-Tag) is a service identifier and is called the I-Component Service Identifier (I- SID). It is added to all PBB frames and contains the service parameters for the quality of service( QoS).