When BGP AD is used for LDP VPLS, with an LDP or GRE transport tunnel, there is no requirement to manually create an SDP. The LDP or GRE SDP can be automatically instantiated using the information advertised by BGP AD. This simplifies the configuration on the service node.
The use of an automatically created GRE tunnel is enabled by creating the PW template used within the service with the parameter auto-gre-sdp. The GRE SDP and SDP binding is created after a matching BGP route has been received.
Enabling LDP on the IP interfaces connecting all nodes between the ingress and the egress builds transport tunnels based on the best IGP path. LDP bindings are automatically built and stored in the hardware. These entries contain an MPLS label pointing to the best next hop along the best path toward the destination.
When two endpoints need to connect and no SDP exists, a new SDP is automatically constructed. New services added between two endpoints that already have an automatically created SDP are immediately used; no new SDP is constructed. The far-end information is learned from the BGP next hop information in the NLRI. When services are withdrawn with a BGP_Unreach-NLRI, the automatically established SDP remains up while at least one service is connected between those endpoints. An automatically created SDP is removed and the resources are released when the only or last service is removed.
The service provider has the option of associating the auto-discovered SDP with a split horizon group using the pw-template-binding option, to control the forwarding between pseudowires and to prevent Layer 2 service loops.
An auto-discovered SDP using a pw-template-binding without a split horizon group configured has similar traffic flooding behavior as a spoke-SDP.