An LSP is a path that is traversed by MPLS packets across an MPLS network. Once the path is established, all subsequent MPLS packets follow the same LSP.
As indicated in MPLS encapsulation and transport, packet-forwarding decisions for an MPLS packet from the ingress PE to the egress PE are made solely on the content of the tunnel label.
When the MPLS packet arrives at the egress PE, the egress PE uses the virtual circuit (VC) or VPN label to deliver the data to the correct end-user.
MPLS permits multiple labels, called label stacks, to be added to an MPLS packet. All traffic-forwarding decisions are based on the outer label of the stack. Multiple labels are used to implement MPLS Layer 2 and Layer 3 VPNs, to send MPLS packets over multiple MPLS networks, and to reroute MPLS packets over TE protection tunnels.