TCP/IP Tutorial and Technical Overview - IBM Redbooks

A.1.2 Benefits
table. This comparison process determines the next hop destination for the
packet. This analysis and classification of the layer-3 header can be
processor-intensive. In a traditional connectionless environment, this activity
occurs at every node along the end-to-end path.
MPLS forwarding model
In an MPLS environment, optimum paths through the network are identified in
advance. Then, as data packets enter the MPLS network, ingress devices use
information in the layer-3 header to assign the packets to one of the
predetermined paths. This assignment is used to append a label referencing the
end-to-end path into the packet. The label accompanies the data packet as it
traverses the network. Subsequent routers along the path use the information in
the label to determine the next hop device. Because these devices only
manipulate information in the label, processor-intensive analysis and
classification of the layer-3 header occurs only at the ingress point.
In additional to reducing the processing requirements on devices in the core of
the network, MPLS has a number of additional advantages over conventional
layer-3 routing, which we describe in the following sections.
Traffic engineering
Traffic engineering is the process of selecting network paths so that the resulting
traffic patterns achieve a balanced utilization of resources.
Routing based on conventional Interior Gateway Protocol (IGP) algorithms might
select network paths that result in unbalanced resource utilization. In these
environments, some network resources are overused, while others are
underused. A limited degree of engineering can be provided by manipulating the
IGP metrics associated with network links. However, this effort is difficult to
manage in environments with a large number of redundant paths.
To achieve the benefits of traffic engineering, MPLS can be used in-conjunction
with IGP algorithms. MPLS provides the ability to specify the specific route data
packets should use to traverse the network. This explicit routing of data packets
ensures that a particular stream of data uses a specific path. By monitoring and
managing these data streams, efficient utilization of network resources can be
achieved. Explicit routing has been available through the source routing options
of traditional IP routing. However, because this is a processor-intensive activity,
its usage has been limited. MPLS makes the efficient use of explicit routing
possible.
Appendix A. Multiprotocol Label Switching 927