Consider the topology below.
---> traffic flow direction ---> ---> PE-1-----P1-----P2-----PE-2 --->
Here PE-1 is the ingress LER and PE-2 is the egress LER. P1 and P2 are LSRs. Now say PHP, (Penultimate Hop Popping) is enabled in these routers. P2 is Pen-Ultimate-Hop here; hence P2 pops the last label (assumming Label-Stack depth is only 1) and sends an IP-Packet to PE-2.
Can PE-2 distinguish between traffic arriving via MPLS-LSP and traffic which is not coming via MPLS-LSP?