Packets traveling along an LSP (see Label switching routers) are identified by its label, the 20-bit, unsigned integer. The range is 0 through 1,048,575. Label values 0 to 15 are reserved and are defined below as follows:
A value of 0 represents the IPv4 Explicit NULL label. It indicates that the label stack must be popped, and the packet forwarding must be based on the IPv4 header. SR OS implementation does not support advertising an explicit-null label value, but can properly process in a received packet.
A value of 1 represents the router alert label. This label value is legal anywhere in the label stack except at the bottom. When a received packet contains this label value at the top of the label stack, it is delivered to a local software module for processing. The actual packet forwarding is determined by the label beneath it in the stack. However, if the packet is further forwarded, the router alert label should be pushed back onto the label stack before forwarding. The use of this label is analogous to the use of the router alert option in IP packets. Because this label cannot occur at the bottom of the stack, it is not associated with a particular network layer protocol.
A value of 2 represents the IPv6 explicit NULL label. It indicates that the label stack must be popped, and the packet forwarding must be based on the IPv6 header. SR OS implementation does not support advertising an explicit-null label value, but can properly process in a received packet.
A value of 3 represents the Implicit NULL label. This is a label that a Label Switching Router (LSR) can assign and distribute, but which never actually appears in the encapsulation. When an LSR would otherwise replace the label at the top of the stack with a new label, but the new label is Implicit NULL, the LSR pops the stack instead of doing the replacement. Although this value may never appear in the encapsulation, it needs to be specified in the Label Distribution Protocol (LDP) or RSVP-TE protocol, so a value is reserved.
A value of 7 represents the Entropy Label Indicator (ELI) which precedes in the label stack the actual Entropy Label (EL) which carries the entropy value of the packet.
A value of 13 represents the Generic-ACH Label (GAL), an alert mechanism used to carry OAM payload in MPLS-TP LSP.
Values 5-6, 8-12, and 14-15 are reserved for future use.
The router uses labels for MPLS, RSVP-TE, LDP, BGP Label Unicast, Segment Routing, as well as packet-based services such as VLL and VPLS.
Label values 16 through 1,048,575 are defined as follows:
label values 16 through 31 are reserved for future use
label values 32 through 18,431 are available for static LSP, MPLS-TP LSP, and static service label assignments. The upper bound of this range, which is also the lower bound of the dynamic label range, is configurable such that the user can expand or shrink the static or dynamic label range.
label values 18,432 through 524,287 (1,048,575 in FP4 system profile B) are assigned dynamically by RSVP, LDP, and BGP control planes for both MPLS LSP and service labels.
label values 524,288 through 1,048,575 are not assigned by SR OS in system profiles other than FP4 profile B, and therefore no POP or SWAP label operation is possible in that range and for those system profiles. However, a PUSH operation, with a label from the full range 32 through 1,048,575 if signaled by some downstream LSR for LSP or service, is supported.
The user can carve out a range of the dynamic label space dedicated for labels of the following features:
Segment Routing Global Block (SRGB) and usable by Segment Routing in OSPF and ISIS.
Reserved Label Block for applications such as SR policy, MPLS forwarding policy, and the assignment of a static label to the SID of a ISIS or OSPF adjacency and adjacency set.