RFC 3032 mentions about reserved labels whose range is from 0-15.Label-0:” Explicit IPv4 Null Label”;
Purpose of this label is same as “implicit Null Label” mentioned below. When we configure “mpls ldp explicit-null” command (default is “implicit null”) on the egress PE, it maps and advertise the prefix in its routing label with label =0 to all LDP neighbors (can be filtered using access list) . The pen-ultimate egress LSR enters this label in LFIB. On label switching, the packet egressing from pen-ultimate LSR to egress LSR will have a label 0. Note: CoS fields (also called EXP fields) in the MPLS label is preserved. The egress LSR on seeing label zero, directly looks into routing table for further processing.
Label-1: “Router-Alert label”;
Can be at any level in the label stack except at bottom. the packet is delivered to s/w for further processing. Same as “Router Alert” option in IP header. When the packet is tag-switched this label is again copied into label stack.
Label-2: “IPv6 Explicit Null Label”;
Same as Label-0, except it is used for IPv6 packets.
Label-3: ” Implicit Null Label”;
Egress LSR advertise a label-3 for the routes which are directly connected and summarized. On received this label by pen-ultimate LSR, it maps “Pop” option under “outgoing field” in LFIB. All packets going out will have the top most label popped out and hence the Cos bits will be removed.
Label-14 will be used as OAM (Operation and Maintainence) Alert Label.HTH, Venkat