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HAPTER

 47: MPLS A

RCHITECTURE

different FECs, based on their FECs, different labels are encapsulated into the 
packets. Later forwarding is based on these labels.

The structure of the label is shown in Figure 120.

Figure 120   Label structure

Label is located between the link layer header and the network layer packet, with 
the length of four bytes. A label contains four fields:

Label: label value, 20 bits.

Exp: three bits, reserved, used for COS.

S: one bit, MPLS supports hierarchical label structure, namely multi-layer label. 
Value 1 refers to the label of bottom layer.

TTL: eight bits, with the same meaning as TTL in IP packet.

1 Label mapping

There are two types of label mapping: label mapping at ingress routers, and label 
mapping in MPLS domain.

The first type of mapping is implemented at Ingress label switching routers (LSR). 
The Ingress LSRs group the incoming packets into multiple FECs based on certain 
principles, and then map corresponding labels to these FECs and record the 
mapping results into the label information base (LIB). In simple words, label 
mapping is to assign a label to a FEC.

The second type is also called incoming label mapping (ILM), that is, to map each 
input label to a series of next hop label forwarding entries (NHLFE). The packets 
are forwarded along the paths based on the mapping results.

2 Label encapsulation

Figure 121 illustrates label encapsulation in different media:

Label

Exp

S

TTL