A variety of technologies are used by carriers to implement Ethernet services. One popular technology is Provider Bridging (PB), which is an extension of the Ethernet bridging standards and is specified in IEEE 802.1ad. Historically Ethernet technology was designed for implementation in the LAN and not for a carrier network. Virtual LAN (VLAN) technology was introduced into theEthernet specifications to allow enterprises to use a single switch to function as if it were multiple virtual switches. Enterprises benefit from the ability to separate traffic at the switch. This could be done to separate traffic from different business units or to provide separation of different traffic types, such as voice and data. Using one switch with VLANs cam reduce CapEx and OpEx for the network. The VLAN concept is a critical element in PB.
PB defines the ability to add a second VLAN Identifier (VID, aka tag) into the Ethernet frame. If the customer connecting to the provider is using VLANs in its network there will be two VIDs in the frame traversing the PB network, where the outer tag (the service tag or S-Tag) is added by the provider and the inner tag (customer tag or C-Tag) is the one the customer added. This stacking of VIDs, VLAN tags, is refereed to as Q-in-Q after the IEEE 802.1Q specification that describes the VLAN concept and its implementation. If the customers is not using VLANs the frames traversing the PB network will have a single provider tag (S-Tag). The service provider uses the S-Tag to separate traffic from different customers, you can consider S-Tag as the customer identifier for the customer Ethernet Virtual Circuit (EVC) in the Metro Ethernet Network (MEN).
PB has been very successful and is used by many carriers to implement Ethernet services in the MAN. The technology is not used in the WAN. PB is a simple expansion of existing Ethernet bridging technology that allows carriers to utilize the VLAN concept to separate customer traffic. Unfortunately the solution is limited by the size of the VID size, which at 12 bits only allows for 4094 unique VIDs. This is a serious limitation for providers with large networks. For E-LAN customers a single VID is used for the entire E-LAN service, which allows the provider to support over 4000 customers in a PB network. For EVPL customers there will be a VID consumed for each pair of endpoints. If a customer has a hub and spoke network with 10 branch (spoke) sites the provider would need to allocate ten VIDs to the customer. If there are several such customers in a PB network the VID pool will be depleted rapidly. For large provider networks using Ethernet technology another level of scaling can be achieved using Provider Backbone Bridging (PBB).
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