Service Access Points (SAPs) are linked to transport tunnels using Service Distribution Points (SDPs). The service architecture allows services to be abstracted from the transport network.
MPLS transport tunnels are signaled using the Resource Reservation Protocol (RSVP-TE) or by the Label Distribution Protocol (LDP). The capability to automatically create an SDP only exists for LDP-based transport tunnels. Using a manually provisioned SDP is available for both RSVP-TE and LDP transport tunnels. For more information about MPLS, LDP and RSVP, see the 7450 ESS, 7750 SR, 7950 XRS, and VSR MPLS Guide.
GRE transport tunnels use GRE encapsulation and can be used with manually provisioned or auto created SDPs.