A.4. Altered STP Behavior in the VPLS Core

The basic idea of the STP improvement is that the fully meshed VPLS core is guaranteed to be loop-free, so STP interoperation with customers should never cause any link cuts in the VPLS mesh. Also in VPLS, STP is deployed on a per-service basis. A provider IP/MPLS VPN backbone can contain many services for different customers. Each VPLS customer has a different service instance in every member PE. Each VPLS service instance in the PE router runs its own STP with its own state machine. In each PE router, every service instance has a VSI to isolate the learning, forwarding, and STP processing from other service instances. Within each VSI, each SAP is a Bridge Port, each spoke-pseudowire is a Bridge Port, and all mesh-pseudowires belong to one Bridge Port. Furthermore, in the VPLS core, all VSIs connected by mesh-pseudowires are one hop away from each other. The STP's behavior is altered in the VSI in the VPLS core.

The first improvement of STP implementation in the VPLS core is the concept of the Primary Bridge. The Primary Bridge is the one VSI in the VPLS core that is closest to the Root Bridge of the entire network. As in the bridged Ethernet networks, when the VPLS service participates in STP with customer switches, the Root Bridge is elected. If the elected Root Bridge is an Ethernet switch in the customer's network, the Primary Bridge is the VSI with a SAP that has the lowest Root Path Cost to the Root Bridge.