More about LSPs and LSP
discovery
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.
MPLS encapsulation
and transport

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.