MPLS Labels
MPLS has two modes: Cell mode and Frame mode. Cell mode is used with
ATM, and the virtual path identifier / virtual circuit identifier (VPI/VCI)
values are used as the MPLS label. Frame mode is used with any Layer 2
protocol that uses frames and inserts a 32-bit label between the Layer 2 and
Layer 3 headers. MPLS routers can do one of three things with a label. They
can impose a label, which means they insert it into the header. They can swap
a label, which means they remove one label and replace it with another. Or
they can pop a label, which means they remove the label. Figure 3-2 shows
the structure of an MPLS label.
Figure 3-2 MPLS Label
Label Number Exp. S TTL
20 bits 3 bits 1
bit
8 bits
The label�fs Number field is 20 bits, the 3 Experimental bits are used to indicate
IP precedence, the 1-bit Bottom of the Stack field indicates whether this is
the last label, and the last 8-bit field indicates a Time to Live value. Multiple
labels can be imposed, such as when using an MPLS VPN, MPLS traffic
engineering, or a combination of the two. Each LSR only acts based on the
top label.
Special Ethertypes are used in an Ethernet header to indicate that an MPLS
label follows, including
�¡ 0x8847.Indicates a unicast labeled IP packet
�¡ 0x8848.Indicates a multicast labeled IP packet