3Com 8807 Configuration Manual page 508

508 C 47: MPLS A
HAPTER RCHITECTURE
different FECs, based on their FECs, different labels are encapsulated into the
packets. Later forwarding is based on these labels.
Label structure
The structure of the label is shown in Figure 120.
Figure 120 Label structure
Label
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.
Label operations
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:
Exp S
TTL