Planet XGS3-24042 User Manual download pdf (Page 647)

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Chapter 79 MPLS Overview

79.1 MPLS Overview

MPLS (Multiprotocol Label Switching), originating from IPv4, was first designed for improving the forwarding

speed. Its core technology can be extended into multiple network protocols, including IPv6 (Internet Protocol

version 6), IPX (Internet Packet Exchange), Appletalk, DECnet, CLNP (Connectionless Network Protocol) and

etc, since the “Multiprotocol” in MPLS means supporting multiple protocols. MPLS technology is a

combination of fast switch and L3 route forwarding hence can satisfy the network requirement of various new

applications.

79.1.1 MPLS Introduction

Forwarding Equivalence Class

MPLS, as a class-based forwarding technology, will put packets with the same forwarding mode into a class

named as FEC (Forwarding Equivalence Class). The same FEC group will be treated with the same way in

MPLS networks. FEC is a group of L3 messages, which will be forwarded along the same path, at the same

priority level, and in the same mode. There are two steps to finish the forwarding process:

 Analyze the packet header and divide packets into FEC

 Map the FEC to the next-hop

In traditional IP forwarding networks, each router will process the same packet with the above two steps. FEC

can include one or more FEC units. All of them are L3 message packets that can be mapped to the same LSP.

At present, there are two types of FEC:

 Address Prefix: Use the Address Prefix to identify a FEC unit, whose length ranges from 0 to the full

address length. Each Address Prefix FEC unit corresponds with a destination subnet.

 Host Address: Use the Host Address to identify a FEC unit, as each unit corresponds with a host

address.

The division rules of FEC is very flexible, which can be any combination of source address, destination

address, source port, destination port, protocol type, VPN and etc. For instance, in the traditional IP

forwarding using the Longest Prefix Match Algorism, all packets targeted at the same destination address

belong to one FEC.

Label

In MPLS networks,,each specific FEC will be encoded at the edge LSR into a label - a short, fixed-length

value, which will be added to the head of packets and turn them into label packets, before they are forwarded.

Besides a segment identifying FEC, labels also include a COS segment, and thus representing FEC,

precedence, and service class as a whole. LSR will divide packets reaching different ports into different FEC

to establish the foundation of VPN. When a LSR creates a new FEC, it will also create a corresponding label,

and advertise it to all peers. LSR maintain both incoming and outgoing labels. To implement load sharing, one

FEC may correspond with multiple labels, but one label can only represent one FEC.

Labels, being carried in packet header, don’t include topology information, and is only locally meaningful. The

label length is 4 bytes. The figure demonstrates its encapsulation structure:

1 2 ... 642 643 644 645 646 647 648 649 650 651 652 ... 720 721

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Planet XGS3-24042 User Manual Page 647

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