1312 CHAPTER 76: MPLS B ASICS CONFIGURATIONn For details about MPLS architecture, refer to RFC 3031 “Multiprotocol LabelSwitching Architecture”.Basic Concepts of MPLS FECAs a forwarding technology based on classification, MPLS groups packets to beforwarded in the same manner into a class called the forwarding equivalence class(FEC). That is, packets of the same FEC are handled in the same way.The classification of FECs is very flexible. It can be based on any combination ofsource address, destination address, source port, destination port, protocol typeand VPN. For example, in the traditional IP forwarding using longest match, allpackets to the same destination belongs to the same FEC.LabelA label is a short fixed length identifier for identifying a FEC. A FEC maycorrespond to multiple labels in scenarios where, for example, load sharing isrequired, while a label can only represent a single FEC.A label is carried in the header of a packet. It does not contain any topologyinformation and is local significant.A label is four octets, or 32 bits, in length. Figure 377 illustrates its format.Figure 377 Format of a labelA label consists of four fields:■ Label: Label value of 20 bits. Used as the pointer for forwarding.■ Exp: For QoS, three bits in length.■ S: Flag for indicating whether the label is at the bottom of the label stack, onebit in length. 1 indicates that the label is at the bottom of the label stack. Thisfield is very useful when there are multiple levels of MPLS labels.■ TTL: Time to live (TTL) for the label. Eight bits in length. This field has the samemeaning as that for an IP packet.Similar to the VPI/VCI in ATM and the DLCI in frame relay, an MPLS label functionsas a connection identifier. If the link layer protocol has a label field like VPI/VCI inATM or DLCI in frame relay, the MPLS label is encapsulated in that field.Otherwise, it is inserted between the data link layer header and the network layerheader as a shim. As such, an MPLS label can be supported by any link layerprotocol.Figure 378 shows the place of a label in a packet.EXP0S19 2223 31Label TTL
