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DES-7200 Configuration Guide Chapter 1 MPLS Configuration
1-5
4 to 15 These values are reserved by the IETF for future usage.
¾ Exp field
The Exp field is currently used to store the QoS information about MPLS. This field is 3 bits.
¾ S mark
The S mark field indicates the stack bottom. It is one bit long. If multiple labels exist, the S bit at
the stack bottom is set to 1 and the S bits of other labels are 0. If only one label exists, the S bit is
directly set to 1.
¾ TTL
Short for Time To Live, the TTL field is 8 bits long. It is similar to the TTL value in IP packet
headers. When a label is first added to an IP packet, the TTL value can be copied from the TTL
field (or HopLimit of IPv6) of the IP packet header. The TTL value of the outer (stack top) label
then decreases by one at every label switching. When MPLS runs on ATM links, the label
encoding methods are different and no TTL field exists. For the corresponding methods and
solutions, refer to RFC 3032.
1.1.2.2 Label Stack
One MPLS packet can have several labels, that is, a label stack. The label that is close to the link
layer header is the top label and the label that is next to the IP header is the bottom label. The
LSR always exchanges labels based on the top label. When multiple labels exist, each label
must be complete and have 32 bits. With the label stack, one MPLS packet can carry multiple
layers of labels. In this manner, the MPLS technology can support hierarchical network systems
and at the same time, support LSPs.
1.1.2.3 Operation Methods of Labels
There are the following basic label operations on MPLS nodes:
¾ Push
Insert a label to the link header and network layer header on an ingress LER or add a new label
to the stack top of an MPLS packet on an intermediate LSR.
¾ Pop
Remove the label of packets on the egress LER to restore the IP packets or remove the top label
on an intermediate LSR to reduce the layers of a label stack.
¾ Swap