53Configuring basic MPLSThe S5500-28SC-HI and S5500-52SC-HI switches do not support MPLS.MPLS overviewMultiprotocol Label Switching (MPLS) enables connection-oriented label switching on connectionless IPnetworks. It integrates both the flexibility of IP routing and the simplicity of Layer 2 switching.MPLS has the following advantages:• MPLS forwards packets according to short- and fixed-length labels, instead of Layer 3 headeranalysis and complicated routing table lookup, enabling highly-efficient and fast data forwardingon backbone networks.• MPLS resides between the link layer and the network layer. It can work over various link layerprotocols (for example, Ethernet), provide connection-oriented services for various network layerprotocols (for example, IPv4 and IPv6), and work with mainstream network technologies.• MPLS is connection-oriented and supports label stack. It can be used to implement various functions,such as VPN, traffic engineering, and QoS.Basic conceptsFECMPLS groups packets with the same characteristics (such as packets with the same destination or serviceclass) into a class, called a "forwarding equivalence class (FEC)". Packets of the same FEC are handledin the same way on an MPLS network. The device supports classifying FECs according to the networklayer destination addresses.LabelA label is a short, fixed length identifier for identifying a single FEC. A label is locally significant andmust be locally unique.Figure 12 Format of a labelA label is encapsulated between the Layer 2 header and Layer 3 header of a packet. A label is four bytesin length and consists of the following fields:• Label—20 bits in length. Label value for identifying a FEC.• Exp—Three bits in length. Reserved field, usually used for Class of Service (CoS).The Exp field marks the CoS of the MPLS packet. It affects the scheduling priority of the packet. Formore information about packet scheduling, see ACL and QoS Configuration Guide.Exp0S19 22 23 31Label TTLLayer 2 header Layer 3 headerLabel Layer 3 data
