Configuring basic MPLS
This chapter describes how to configure basic MPLS.
Multiprotocol Label Switching (MPLS) enables connection-oriented label switching on connectionless IP
networks. It integrates both the flexibility of IP routing and the level of simplicity of Layer 2 switching.
MPLS has the following advantages:
MPLS forwards packets according to short- and fixed-length labels, instead of Layer 3 header
analysis and complicated routing table lookup, enabling highly-efficient and fast data forwarding
on backbone networks.
MPLS resides between the link layer and the network layer. It can operate over various link layer
protocols (for example, PPP, ATM, frame relay, and Ethernet), provide connection-oriented services
for various network layer protocols (for example, IPv4, IPv6, and IPX), and operate with mainstream
MPLS is connection-oriented and supports label stack. It can be used to implement various functions,
such as VPN, traffic engineering, and QoS.
This section describes the basic concepts of MPLS.
MPLS groups packets with the same characteristics (such as packets with the same destination or service
class) into a class called a "forwarding equivalence class (FEC)." Packets of the same FEC are handled
in the same way on an MPLS network. The device supports classifying FECs according to the network
layer destination addresses.
A label is a short, fixed length identifier for identifying a single FEC. A label is locally significant and
must be locally unique.
Figure 12 Format of a label
Layer 2 header
A label is encapsulated between the Layer 2 header and Layer 3 header of a packet. A label is four bytes
in 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 CoS.
Layer 3 header
Layer 3 data