IEEE 802.1ah is an amendment to theIEEE 802.1Qnetworking standard which adds support forProvider Backbone Bridges. It includes an architecture and a set of protocols for routing over a provider's network, allowing interconnection of multipleprovider bridge networks without losing each customer's individually definedVLANs. It was initially created byNortel before being submitted to theIEEE 802.1 committee for standardization. The final version was approved by theIEEE in June 2008 and has been integrated into IEEE 802.1Q-2011.

The now-ubiquitousEthernet was initially defined as alocal 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 inIEEE 802.1ad standard. In the 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 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.

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:^[1][2]

Backbone Destination Address (B-DA): 48 bits

Backbone destination MAC address.

Backbone Source Address (B-SA): 48 bits

Backbone source MAC address.

Backbone EtherType: 16 bits

Backbone EtherType. 0x88A8 indicates Service VLAN tag identifier (S-Tag).

Backbone Flags: 4 bits

Backbone flag bits. They are (in order, from most significant to least significant):

Priority Code Point (PCP): 3 bits

Priority Code Point.

Drop Eligible Indicator (DEI): 1 bit

Drop Eligible Indicator.

Backbone VLAN ID (B-VID): 12 bits

Backbone VLAN identifier.

MIM EtherType: 16 bits

MIM EtherType. 0x887E indicates Provider Backbone Bridges.

MIM Flags: 8 bits

Flag bits. They are (in order, from most significant to least significant):

Priority Code Point (PCP): 3 bits

Priority Code Point.

Drop Eligible Indicator (DEI): 1 bit

Drop Eligible Indicator.

No Customer Access (NCA): 1 bit

E.g. OAM frames.

Reserved1 (Res1): 1 bit

Reserved.

Reserved2 (Res2): 2 bits

Reserved.

MIM Service Identifier (I-SID): 24 bits

Service Instance VLAN ID.

Customer Destination Address (C-DA): 48 bits

Customer destination MAC address.

Customer Source Address (C-SA): 48 bits

Customer source MAC address.

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 theIEEE 802.1Qay PBB-TE standard, which was standardized in 2009.^[3]

PBB is sometimes referred to as MAC-in-MAC.^[why?]

• Carrier Ethernet
• IEEE 802.1
• IEEE 802.1aq, Shortest Path Bridging
• Metro Ethernet

• http://www.ieee802.org/1/pages/802.1ah.html
• The right transport foundation for carrier Ethernet services

• IEEE 802
• Ethernet standards
• Telecommunication protocols

• Articles with short description
• Short description is different from Wikidata
• Wikipedia articles needing clarification from January 2023