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1-2
MPLS integrates both Layer 2 fast switching and Layer 3 routing and forwarding, satisfying the
networking requirements of various new applications.
For details about the MPLS architecture, refer to RFC 3031 “Multiprotocol Label Switching
Architecture”.
Basic Concepts of MPLS
FEC
As a forwarding technology based on classification, MPLS groups packets to be forwarded in the same
manner into a class called a forwarding equivalence class (FEC). That is, packets of the same FEC are
handled in the same way on an MPLS network.
The classification of FECs is very flexible. It can be based on any combination of source address,
destination address, source port, destination port, protocol type and Virtual Private Network (VPN). For
example, in traditional IP forwarding using the longest match algorithm, all packets to the same
destination belong to the same FEC.
Label
A label is a short, fixed length identifier for identifying a FEC. A FEC may correspond to multiple labels in
scenarios where, for example, load sharing is required, while a label can only represent a single FEC.
A label is carried in the header of a packet. It does not contain any topology information and is local
significant.
A label is four octets, or 32 bits, in length. Figure 1-1 illustrates its format.
Figure 1-1
Format of a label
A label consists of four fields:
z
Label: Label value of 20 bits. Used as the pointer for forwarding.
z
Exp: For QoS, three bits in length.
z
S: Flag for indicating whether the label is at the bottom of the label stack, one bit in length. 1
indicates that the label is at the bottom of the label stack. This field is very useful when there are
multiple levels of MPLS labels.
z
TTL: Time to live (TTL) for the label. Eight bits in length. This field has the same meaning as that for
an IP packet.
Similar to the VPI/VCI in ATM and the DLCI in frame relay, an MPLS label functions as a connection
identifier. If the link layer protocol has a label field like VPI/VCI in ATM 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 layer header as a shim. As such, an MPLS label can be supported by any link layer protocol.
Figure 1-2 shows the place of a label in a packet.