MPLS Guide MPLS and RSVP-TE
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3.2.2 MPLS Label Stack
Routers that support MPLS are known as Label Edge Routers (LERs) and Label
Switch Routers (LSRs). MPLS requires a set of procedures to enhance network layer
packets with label stacks, which turns them into labeled packets. In order to initiate,
transmit, or terminate a labeled packet on a particular data link, an LER or LSR must
support the encoding technique which, when given a label stack and a network layer
packet, produces a labeled packet.
In MPLS, packets can carry not just one label, but a set of labels in a stack. An LSR
can swap the label at the top of the stack, pop the stack (that is, remove the top
label), or swap the label and push one or more labels onto the stack. The processing
of a labeled packet is completely independent of the level of hierarchy. The
processing is always based on the top label, without regard for the possibility that
other labels may have been above it in the past or that other labels may be below it
at present.
As described in RFC 3032, MPLS Label Stack Encoding, the label stack is
represented as a sequence of “label stack entries”. Each label stack entry is
represented by 4 octets. Figure 1 shows the structure of a label and Table 2
describes the fields. Figure 2 shows the label placement in a packet.
Figure 1 Label Structure
Table 2 Packet/Label Field Description
Field Description
Label This 20-bit field carries the actual value (unstructured) of the label.
Exp This 3-bit field is reserved for experimental use. It is currently used for Class
of Service (CoS).
S This bit is set to 1 for the last entry (bottom) in the label stack and 0 for all
other label stack entries.
TTL This 8-bit field is used to encode a time-to-live value.
0123456789012345678901234567890
01
Label 1 Exp S TTL
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