Labels 0 through 15 are reserved labels. An LSR cannot use them in the normal case for forwarding packets. An LSR assigns a specific function to each of these labels. Label 0 is the explicit NULL label, whereas label 3 is the implicit NULL label. Label 1 is the router alert label, whereas label 14 is the OAM alert label. The other reserved labels between 0 and 15 have not been assigned yet.
Implicit NULL Label
The implicit NULL label is the label that has a value of 3. An egress LSR assigns the implicit NULL label to a FEC if it does not want to assign a label to that FEC, thus requesting the upstream LSR to perform a pop operation. In the case of a plain IPv4-over-MPLS network, such as an IPv4 network in which LDP distributes ...