Special MPLS Labels
Some of the reserved labels (in the 0 through 15 range) have well-defined meanings. For more complete details, see RFC 3032, MPLS Label Stack Encoding.
0, IPv4 Explicit Null label—This value is legal only when it is the sole label entry (no label stacking). It indicates that the label must be popped upon receipt. Forwarding continues based on the IP version 4 (IPv4) packet.
1, Router Alert label—When a packet is received with a top label value of 1, it is delivered to the local software module for processing.
2, IPv6 Explicit Null label—This value is legal only when it is the sole label entry (no label stacking). It indicates that the label must be popped on receipt. Forwarding continues based on the IP version 6 (IPv6) packet.
3, Implicit Null label—This label is used in the control protocol (LDP or RSVP) only to request label popping by the downstream router. It never actually appears in the encapsulation. Labels with a value of 3 should not be used in the data packet as real labels. No payload type (IPv4 or IPv6) is implied with this label.
4 through 6—Unassigned.
7, Entropy label indicator—This label is used when an Entropy label is in the label stack and precedes the Entropy label.
8 through 15—Unassigned.
Special labels are commonly used between the egress and penultimate routers of an LSP. If the LSP is configured to carry IPv4 packets only, the egress router might signal the penultimate router to use 0 as a final-hop label. If the LSP is configured to carry IPv6 packets only, the egress router might signal the penultimate router to use 2 as a final-hop label.
The egress router might simply signal the penultimate router to use 3 as the final label, which is a request to perform penultimate-hop label popping. The egress router will not process a labeled packet; rather, it receives the payload (IPv4, IPv6, or others) directly, reducing one MPLS lookup at egress.
For label-stacked packets, the egress router receives an MPLS label packet with its top label already popped by the penultimate router. The egress router cannot receive label-stacked packets that use label 0 or 2. It typically requests label 3 from the penultimate router.