Multiprotocol Label Switching:
Multiprotocol Label Switching (MPLS) is a data-carrying
mechanism capable of emulating properties of a circuit-switched
network over a packet-switched network. MPLS operates at a
layer that is generally considered to lie between the traditional
definitions of layer 2 and layer 3, and thus is often referred to as
a layer 2.5 protocol. It was designed to provide a unified datacarrying
service for both circuit-based clients and packetswitching
clients which provide a datagram service model. It can
be used to carry many different kinds of traffic, including IP
packets, as well as native ATM, SONET, and Ethernet frames.
MPLS has the capability to displace ATM and Frame Relay
networks in service-provider backbones and some large
enterprise private WANs, while providing backward compatibility with existing networks.
A number of different WAN technologies were previously
deployed with essentially identical goals, such as Frame Relay
and ATM. MPLS takes up where Frame Relay and ATM leave off,
borrowing what works well from those technologies, while leaving
behind concepts that have become outdated.
For example, MPLS recognizes that ATM cells are not needed in
the core of modern networks, as modern optical networks are
capable of transmitting full-length 1500-byte IP packets without
the types of delays that the 53 byte cells of ATM were designed to prevent.
At the same time, it attempts to preserve the traffic engineering
and out-of-band control that made Frame Relay and ATM attractive for deployment in large scale networks.
The MPLS Core:
Multiprotocol Label Switching is designed to make up the core of
a service provider (or large corporate) WAN. At the edge of the
MPLS core are Label Edge Routers (LERs). LERs prepare
packets for entry, or ingress, into the MPLS core and exit, or
egress, from the core. Within the core are Label Switch Routers
(LSRs), which switch packets on their journey through the core.
(See Figure 1-26). The specific responsibilities of LERs and
LSRs are described in the next section.
Before examining how MPLS works, it is important to define some basic concepts used in MPLS networks.
Forward Equivalency Class (FEC)—A FEC describes a set of
packets with similar or identical characteristics that may be
transported together through a network it the same way. Quality
of service parameters are often used for defining FECs.
Label—Used to identify a direct path between two individual
nodes in an MPLS network.
Label Information Base (LIB)——A table maintained by LSRs and LERs that maps FECs to labels.
Label Switched Path (LSP)—A series of labels that indicates
the end-to-end path of a packet through an MPLS network. It is sometimes referred to as an MPLS tunnel.