H3C Technologies H3C S12500-X Series Switches - Manual (Page 12)

1
Configuring basic MPLS
Multiprotocol Label Switching (MPLS) provides connection-oriented label switching over connectionless IP
backbone networks. It integrates both the flexibility of IP routing and the simplicity of Layer 2 switching.
Overview
MPLS has the following advantages:
• High speed and efficiency—MPLS uses short- and fixed-length labels to forward packets, avoiding
complicated routing table lookups.
• Multiprotocol support—MPLS resides between the link layer and the network layer. It can work over
various link layer protocols (for example, PPP, ATM, frame relay, and Ethernet) to provide
connection-oriented services for various network layer protocols (for example, IPv4, IPv6, and IPX).
• Good scalability—The connection-oriented switching and multi-layer label stack features enable
MPLS to deliver various extended services, such as VPN, traffic engineering, and QoS.
Basic concepts
FEC
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.
Label
A label uniquely identifies a FEC and has local significance.
Figure 1 Format of a label
A label is encapsulated between the Layer 2 header and Layer 3 header of a packet. It is four bytes long
and consists of the following fields:
• Label—20-bit label value.
• TC—3-bit traffic class, used for QoS. It is also called "Exp."
• S—1-bit bottom of stack flag. A label stack can comprise multiple labels. The label nearest to the
Layer 2 header is called the "top label," and the label nearest to the Layer 3 header is called the
"bottom label." The S field is set to 1 if the label is the bottom label and set to 0 if not.
• TTL—8-bit time to live field used for routing loop prevention.