IEEE 802.1ah-2008

Provider Backbone Bridges (PBB) or IEEE 802.1ah-2008 is a set of architecture and protocols for routing of a customer network over a providers network allowing interconnection of multiple Provider Bridge Networks without losing each customers individually defined VLANS. It was initially created as a proprietary extension by Nortel before being submitted to the IEEE 802.1 committee for standardization. The final standard was approved by the IEEE in June 2008.

= History =Ethernet was initially defined as a LAN technology to interconnect the computer within a small organization. Over the years, Ethernet has become such a popular technology that it became the default OSI Layer 2 mechanism for any data transport. This created a need for extending the Ethernet from a customer LAN bridging domain to service provider MAN or the Provider bridging domain. For this, initially a 4 byte S-Tag was added to the Ethernet frame in IEEE 802.1ad standard. In service provider domain, switching was based on S-Tag and destination MAC address, and C-tag was used to create virtual LAN within the customer domain. This technology is also known as QinQ or Q-tunneling.

QinQ does not offer true separation of customer and provider domains, but is merely a way to overcome the limitations on the VLAN identifier space. It can also help in separation of the customer and provider control domains when used with other features like control protocol tunneling or Per VLAN Spanning Tree (like spanning trees etc.). There is still the problem of having too little control on the MAC addresses, since QinQ forwarding is still based on the customer destination addresses. Thus, better mechanisms are needed.

=Principle of Operation=

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 (six bytes)
** Backbone source address (six bytes)
** EtherType 0x88A8 (two bytes)
** B-TAG/B-VID (two bytes), this is the backbone VLAN indicator
* Service encapsulation, that has:
** EtherType 0x88E7 (two bytes)
** Flags that contain priority, DEI and indication, if there are no customer addresses present (e.g. OAM frames).
** I-SID, the service identifier (three bytes)

* Original customer frame
** Customer source address (six bytes)
** Customer destination 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 address. It also define 12 bit B-VID (backbone VLAN ID)and 24 bit I-SID (Service Instance VLAN ID). The bridges in PBB domain switches based on 60 bit B-VID and B-DA value. Bridges learn based on the B-SA and ingress port value and hence is completely unaware of the customer MAC addresses. I-SID allows to distinguish the services within a PBB domain.

PBB is the foundation for the under development IEEE 802.1Qay PBB-TE standard.

ee also

*Provider Backbone Bridge Traffic Engineering or (PBB-TE)

References

External links

*http://www.ieee802.org/1/pages/802.1ah.html
* [http://telephonyonline.com/ethernet/technology/transport_ethernet_services_060907/ The right transport foundation for carrier Ethernet services]