The 7705 SAR uses RSVP-TE and LDP protocols for label forwarding, For packet-based services such as VLL, the 7705 SAR uses T-LDP for signaling PW labels between peer nodes.
Packets traveling along an LSP are identified by the packet label, which is the 20-bit, unsigned integer (see Label Edge and Label Switch Routers). The range is 0 through 1 048 575. Label values 0 to 15 are reserved and are defined below:
A value of 0 represents the IPv4 Explicit NULL label. This label value is legal only at the bottom of the label stack if the label stack is immediately followed by an IPv4 header, in which case the packet forwarding is based on the IPv4 header. If the IPv4 Explicit NULL label is not at the bottom of the label stack, then the packet forwarding is based on the subsequent label.
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 be 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. An LER advertises this when it is requesting penultimate hop popping (PHP) and expecting unlabeled packets. The label value 3 should never appear in the label stack.
A value of 7 represents the entropy label indicator (ELI). The ELI is a special-purpose MPLS label that indicates that the entropy label (EL) follows it in the stack.
Values 4 through 6 and 8 through 15 are reserved for future use.
Table: Ingress Label Values (Pop Labels) lists the label ranges available for use by ingress labels (pop labels).
Label Values |
Description |
|---|---|
16 through 31 |
Reserved for future use |
32 through 1023 |
Available for static outer LSP tunnel label assignment |
1024 through 2047 |
Reserved for future use |
|
2048 through 18 431 1 |
Statically assigned for services (inner pseudowire label) |
32 768 through 131 071 |
Dynamically assigned for both MPLS and services |
131 072 through 1 048 575 |
Reserved for future use |
Table: Egress Label Values (Push Labels) lists the label ranges available for use by egress labels (push labels).
Label Values |
Description |
|---|---|
16 through 1 048 575 |
Can be used for static LSP tunnel and static PW labels |
16 through 1 048 575 |
Can be dynamically assigned for both MPLS tunnel labels and PW labels |