PLANET XGS3-24042 - Manual (Page 647)

79-1
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: