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DES-7200 Configuration Guide Chapter 1 MPLS Configuration
1-4
Reserved Label Value Meaning
0
Indicates the IPv4 explicit null label. According to RFC 3032,
the 0 label must be at the stack bottom. This means that the
label should be popped out and the packets should then be
forwarded according to destination IP addresses. RFC 4182
modifies the description of 0 label in RFC 3032. For the
received packets with 0 label, the router directly pops out the
label and determines the forwarding action based on the
contents after 0 label. If another label follows, the router
forwards the packets according to the label; if the packets are
IPv4 packets, the router forwards them according to their
destination IP addresses.
1
Indicates the router alert label. This label is not allowed at the
bottom of the label stack. When receiving packets with the
router alert label, the router must send the packets to the local
software module for processing. The actual forwarding of the
packets must be based on the labels that follow the router
alert label. Before the forwarding, however, the router alert
label must be added to the label stack again. This option is
similar to the Router Alert Option of IP packets. You can use
this option to configure the LSRs on each hop to check MPLS
packets.
2
Indicates the IPv6 explicit null label. According to RFC 3032,
the 2 label must be at the stack bottom. This means that the
label should be popped out and the packets should then be
forwarded according to destination IP addresses. RFC 4182
modifies the description of 2 label in RFC 3032. For the
received packets with 2 label, the router directly pops out the
label and determines the forwarding action based on the
contents after 2 label. If another label follows, the router
forwards the packets according to the label; if the packets are
IPv4 packets, the router forwards them according to their
destination IP addresses.
3
Indicates the implicit null label. This label can be distributed
by the label distribution protocol (LDP) but can never be
transmitted in the label stacks of MPLS packets. When an
LSR exchanges MPLS packets, the router pops out the label
of the stack top rather than replaces the label if the label to be
replaced at the stack top is 3. The implicit null label is used in
the Penultimate Hop Popping (PHP) function.