History
The now-ubiquitous Ethernet was initially defined as a local area network (LAN) technology to interconnect the computers within a small organization in which these host computers were very close in proximity to each other. Over the years, Ethernet has become such a popular technology that it became the default Data Link Layer (OSI Layer 2) mechanism for data transport. This created a need for extending the Ethernet from a customer LAN bridging domain to service provider MAN, also known as the Provider bridging domain. For this, a 4 byte S-Tag or Service Tag, a type of Virtual LAN tag, was added to the header of the Ethernet frame inDescription
The idea of PBB is to offer complete separation of customer and provider domains. For this purpose, a new Ethernet header has been defined. This header may take multiple different forms, but the main components of the header are: * Backbone component, that has: ** Backbone destination address (B-DA) (six bytes) ** Backbone source address (B-SA) (six bytes) ** EtherType 0x88A8 (two bytes) ** B-TAG/B-VID (two bytes), this is the backbone VLAN indicator * Service encapsulation, which has: ** EtherType 0x88E7 (two bytes) ** Flags that contain priority, Drop Eligible Indicator (DEI) and No Customer Address (NCA) indication (e.g. OAM frames). ** I-SID, the service identifier (three bytes) * Original customer frame ** Customer destination address (six bytes) ** Customer source address (six bytes) ** EtherType 0x8100 (two bytes) ** Customer VLAN identifier (two bytes) ** EtherType (e.g. 0x0800) ** Customer payload PBB defines a 48-bit B-DA and 48-bit B-SA to indicate the backbone source and destination MAC addresses. It also defines a 12-bit B-VID (backbone VLAN ID) and 24-bit I-SID (Service Instance VLAN ID). The bridges in the PBB domain switch based on the B-VID and B-DA values, which contain 60 bits total. Bridges learn based on the B-SA and ingress port value and hence is completely unaware of the customer MAC addresses. I-SID allows distinguishing the services within a PBB domain. PBB is the foundation for the IEEE 802.1Qay PBB-TE standard, which was standardized in 2009. PBB is sometimes referred to as MAC-in-MAC.See also
* Carrier Ethernet * IEEE 802.1 * IEEE 802.1aq, Shortest Path Bridging * Metro EthernetReferences
External links
*http://www.ieee802.org/1/pages/802.1ah.html