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In computer networking and telecommunications, MultiProtocol Label Switching (MPLS) is a data-carrying mechanism which emulates some properties of a circuit-switched network over a packet-switched network. more...
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MPLS operates at an OSI Model layer that is generally considered to lie between traditional definitions of Layer 2 (data link layer) and Layer 3 (network layer), and thus is often referred to as a "Layer 2.5" protocol. It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching 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.
This is a basic introduction. To obtain more detail on MPLS see External Links.
Background
A number of different technologies were previously deployed with essentially identical goals, such as frame relay and ATM. MPLS is now replacing these technologies in the marketplace, mostly because it is better aligned with current and future technology needs.
In particular, MPLS dispenses with the cell-switching and signalling-protocol baggage of ATM. MPLS recognizes that small ATM cells are not needed in the core of modern networks, since modern optical networks (as of 2001) are so fast (at 10 Gbit/s and well beyond) that even full-length 1500 byte packets do not incur significant real-time queuing delays (the need to reduce such delays, to support voice traffic, having been the motivation for the cell nature of ATM).
At the same time, it attempts to preserve the traffic engineering and out-of-band control that made frame relay and ATM attractive for deploying large scale networks.
MPLS was originally proposed by a group of engineers from Cisco Systems, Inc.; it was called "Tag Switching" when it was a Cisco proprietary proposal, and was renamed "Label Switching" when it was handed over to the IETF for open standardization.
One original motivation was to allow the creation of simple high-speed switches, since for a significant length of time it was impossible to forward IP packets entirely in hardware. However, advances in VLSI have made such devices possible. The systemic advantages of MPLS, such as the ability to support multiple service models, do traffic management, etc, remain.
How MPLS works
MPLS works by prepending packets with an MPLS header, containing one or more 'labels'. This is called a label stack.
 Each label stack entry contains four fields:
a 20-bit label value.;
a 3-bit field for QoS priority.;
a 1-bit bottom of stack flag. If this is set, it signifies the current label is the last in the stack.;
an 8-bit TTL (time to live) field.;
These MPLS labeled packets are forwarded (switched is the correct term) after a Label Lookup/Switch instead of a lookup into the IP table. Label Lookup and Label Switching may be faster than usual RIB lookup because it can take place directly into fabric and not CPU.
Read more at Wikipedia.org
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