MPLS Label Stack
Page 26 7450 ESS MPLS Guide
Label Values
Packets travelling along an LSP (see Label Switching Routers on page 27) are identified by its
label, the 20-bit, unsigned integer. The range is 0 through 1,048,575. Label values 0-15 are
reserved and are defined below as follows:
• A value of 0 represents the IPv4 Explicit NULL Label. This Label value is legal only at
the bottom of the Label stack. It indicates that the Label stack must be popped, and the
packet forwarding must be based on the IPv4 header.
• 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. Since this Label cannot occur at the bottom of the stack, it is not
associated with a particular network layer protocol.
• 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), so a value is reserved.
• Values 4-15 are reserved for future use.
The router uses labels for MPLS, RSVP-TE, and LDP, 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 1,023 are available for static LSP label assignments.
• Label values 1,024 through 2,047 are reserved for future use.
• Label values 2,048 through 18,431 are available for static service label assignments
• Label values 18,432 through 262,143 (131,071 in chassis modes lower than D) are
assigned dynamically by RSVP, LDP, and BGP control planes for both MPLS LSP and
service labels.
• Label values 262,144 (131,072 in chassis modes lower than D) through 1,048,575 are
reserved for future use.
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