Multi-Protocol Label Switching (MPLS)
was originally presented as a way of improving the forwarding
speed of routers but is now emerging as a crucial standard
technology that offers new capabilities for large scale
IP networks. Traffic engineering, the ability of network
operators to dictate the path that traffic takes through
their network, and Virtual Private Network support are
examples of two key applications where MPLS is superior
to any currently available IP technology.
Although MPLS was conceived as being
independent of Layer 2, much of the excitement generated
by MPLS revolves around its promise to provide a more effective
means of deploying IP networks across ATM-based WAN backbones.
The Internet Engineering Task Force is developing MPLS
with draft standards expected by the end of 1998. MPLS
is viewed by some as one of the most important network
developments of the 1990's. This article will explain why
MPLS is generating such interest.
The essence of MPLS is the generation
of a short fixed-length label that acts as a shorthand
representation of an IP packet's header. This is much the
same way as a ZIP code is shorthand for the house, street
and city in a postal address, and the use of that label
to make forwarding decisions about the packet. IP packets
have a field in their 'header' that contains the address
to which the packet is to be routed. Traditional routed
networks process this information at every router in a
packet's path through the network (hop by hop routing).
In MPLS, the IP packets are encapsulated
with these labels by the first MPLS device they encounter
as they enter the network. The MPLS edge router analyses
the contents of the IP header and selects an appropriate
label with which to encapsulate the packet. Part of the
great power of MPLS comes from the fact that, in contrast
to conventional IP routing, this analysis can be based
on more than just the destination address carried in the
IP header. At all the subsequent nodes within the network
the MPLS label, and not the IP header, is used to make
the forwarding decision for the packet. Finally, as MPLS
labeled packets leave the network, another edge router
removes the labels.
In MPLS terminology, the packet handling
nodes or routers are called Label Switched Routers (LSRs).
The derivation of the term should be obvious; MPLS routers
forward packets by making switching decisions based on
the MPLS label. This illustrates another of the key concepts
in MPLS. Conventional IP routers contain routing tables
which are looked up using the IP header from a packet to
decide how to forward that packet. These tables are built
by IP routing protocols (e.g., RIP or OSPF) which carry
around IP reachability information in the form of IP addresses.
In practice, we find that forwarding (IP header lookup)
and control planes (generation of the routing tables) are
tightly coupled. Since MPLS forwarding is based on labels
it is possible to cleanly separate the (label-based) forwarding
plane from the routing protocol control plane. By separating
the two, each can be modified independently. With such
a separation, we don't need to change the forwarding machinery,
for example, to migrate a new routing strategy into the
There are two broad categories of LSR.
At the edge of the network, we require high performance
packet classifiers that can apply (and remove) the requisite
labels: we call these MPLS edge routers. Core LSRs need
to be capable of processing the labeled packets at extremely
This is an abstract of the MPLS article
contained in techguide.com. The complete article examines
MPLS and the opportunities it offers to users and also
to the service providers who are designing and engineering
the next generation of IP networks. It also describes why
new carrier-class edge devices will become a key component
in the provisioning of future network services.
View more information in protocols.com
View and print the entire article in techguide.com.