Operation Manual – MPLS Basics
H3C S9500 Series Routing Switches
Chapter 1 MPLS Basics Configuration
1-2
Internet packet exchange (IPX), and connectionless network protocol (CLNP). That is
what the term multiprotocol means.
MPLS integrates both Layer 2 fast switching and Layer 3 routing and forwarding,
satisfying the requirements of various new applications for network performance.
Note:
For details about MPLS architecture, refer to RFC 3031 “Multiprotocol Label Switching
Architecture”.
1.1.1 Basic Concepts of MPLS
I. FEC
As a forwarding technology based on classification, MPLS groups packets to be
forwarded 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 of source
address, destination address, source port, destination port, protocol type and VPN. For
example, in the traditional IP forwarding using longest match, all packets to the same
destination belongs to the same FEC.
II. 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.
