Operation Manual – MPLSH3C S9500 Series Routing Switches Chapter 1 MPLS Architecture1-2based on network layer address. Packets of the same FEC are processed in the sameway in MPLS network.1.2.2 LabelI. Label definitionA label is a locally significant short identifier with fixed length, which is used to identify aFEC. When reaching at MPLS network ingress, packets are divided into different FECs,based on their FECs, different labels are encapsulated into the packets. Laterforwarding is based on these labels.II. Label structureThe structure of the label is shown in Figure 1-1.Label Exp S TTLFigure 1-1 Label structureLabel is located between the link layer header and the network layer packet, with thelength 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 1refers to the label of bottom layer.TTL: eight bits, with the same meaning as TTL in IP packet.III. Label operations1) Label mappingThere are two types of label mapping: label mapping at ingress routers, and labelmapping in MPLS domain.The first type of mapping is implemented at Ingress label switching routers (LSR). TheIngress LSRs group the incoming packets into multiple FECs based on certainprinciples, and then map corresponding labels to these FECs and record the mappingresults into the label information base (LIB). In simple words, label mapping is to assigna label to a FEC.The second type is also called incoming label mapping (ILM), that is, to map each inputlabel to a series of next hop label forwarding entries (NHLFE). The packets areforwarded along the paths based on the mapping results.2) Label encapsulationFigure 1-2 illustrates label encapsulation in different media:
