Configuring Basic Mpls; Overview; Basic Concepts - HP HSR6800 Configuration Manual

Configuring basic MPLS

Overview

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 work 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 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 concepts

This section describes the basic concepts of MPLS.
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. The device supports classifying FECs according
to the network layer destination addresses.
Label
A label is a short, fixed length identifier for identifying a single FEC. A label is locally significant and
must be locally unique.
Figure 1 Format of a label
0
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.
•
S—One bit in length. MPLS supports multiple levels of labels. This field indicates whether a
label is at the bottom of the label stack. A value of 1 indicates that the label is at the bottom of
the label stack.
•
TTL—Eight bits in length. Like the homonymous IP header field, it is used to prevent loops.
Label
Label Layer 3 header
19 22 23
Exp S TTL
Layer 3 data
1
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