Reserved Labels - Dell PowerConnect J-EX4200-24T Software Manual

Dell PowerConnect J-Series Ethernet Switch Complete Software Guide for Junos OS

Reserved Labels

MPLS Label Operations on J-EX Series Switches
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Figure 81: Label Encoding
Labels range from 0 through 1,048,575. Labels 0 through 999,999 are for internal use.
Some of the reserved labels (in the 0 through 15 range) have well-defined meanings.
The following reserved labels are used by J-EX Series switches:
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 on 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.
3, Implicit Null label—This label is used in the control protocol (RSVP) only to request
label popping by the downstream switch. It never actually appears in the encapsulation.
Labels with a value of 3 must not be used in the data packet as real labels. No payload
type (IPv4 or IPv6) is implied with this label.
J-EX Series switches support the following label operations:
Push
Pop
Swap
The push operation affixes a new label to the top of the IP packet. For IPv4 packets, the
new label is the first label. The time to live (TTL) field value in the packet header is derived
from the IP packet header. The push operation cannot be applied to a packet that already
has an MPLS label.