What is required for deploying MPLS Layer 3 VPN services between PEs?

For MPLS Layer 3 VPN services, a Label Switched Path (LSP) must exist between the Provider Edge (PE) routers. This LSP, typically built using LDP or RSVP-TE, provides the underlying label-switched forwarding plane for VPN traffic. Without an LSP, the PE routers cannot exchange labeled VPN packets. The RD (Route Distinguisher) and RT (Route Target) values do not need to match between PEs for basic connectivity; RDs ensure unique prefixes within a VRF, and RTs control route import/export policies. While RTs define which routes are exchanged, their specific values don't directly enable the underlying label-switched forwarding. The import and export RT values within a single VRF are often different to allow flexible routing policies.