MPLS supports a range of access technologies, including T1/E1, ATM, Frame Relay, and DSL.
Packet-forwarding decisions are made solely on the contents of this label, without the need to examine the packet itself.
It can be used to carry many different kinds of traffic, including IP packets, as well as native Asynchronous Transfer Mode (ATM), Frame Relay, Synchronous Optical Networking (SONET) or Ethernet.
A number of different technologies were previously deployed with essentially identical goals, such as Frame Relay and ATM.
The similarity between Frame Relay, ATM, and MPLS is that at each hop throughout the network, the label value in the header is changed.
[a] At the same time, MPLS attempts to preserve the traffic engineering (TE) and out-of-band control that made Frame Relay and ATM attractive for deploying large-scale networks.
[9] Some time later it was recognized that the work on threaded indices by Girish Chandranmenon and George Varghese had invented the idea of using labels to represent destination prefixes that was central to tag switching.
[10] One original motivation was to allow the creation of simple high-speed switches since for a significant length of time it was considered impractical to forward IP packets entirely in hardware.
The current advantages of MPLS primarily revolve around the ability to support multiple service models and perform traffic management.
MPLS also offers a robust recovery framework[11] that goes beyond the simple protection rings of synchronous optical networking (SONET/SDH).
Alternatively, under penultimate hop popping this function may instead be performed by the LSR directly connected to the LER.
In many respects, LSPs are not different from permanent virtual circuits (PVCs) in ATM or Frame Relay networks, except that they are not dependent on a particular layer-2 technology.
The egress router must, therefore, have routing information for the packet's payload since it must forward it without the help of label lookup tables.
This is useful in cases where the egress router has many packets leaving MPLS tunnels and thus spends significant CPU resources on these transitions.
The path begins at an LER, which makes a decision on which label to prefix to a packet based on the appropriate FEC.
Note that LSPs are unidirectional; they enable a packet to be label switched through the MPLS network from one endpoint to another.
Since bidirectional communication is typically desired, the aforementioned dynamic signaling protocols can automatically set up a separate LSP in the opposite direction.
When link protection is considered, LSPs can be categorized as primary (working), secondary (backup) and tertiary (LSP of last resort).
The paths an LSR knows can be defined using explicit hop-by-hop configuration, or are dynamically routed by the Constrained Shortest Path First (CSPF) algorithm, or are configured as a loose route that avoids a particular IP address or that is partly explicit and partly dynamic.
In the event of a network element failure when recovery mechanisms are employed at the IP layer, restoration may take several seconds which may be unacceptable for real-time applications such as VoIP.
[31][32][33] In contrast, MPLS local protection meets the requirements of real-time applications with recovery times comparable to those of shortest path bridging networks or SONET rings of less than 50 ms.[31][33][34] MPLS can make use of existing ATM network or Frame Relay infrastructure, as its labeled flows can be mapped to ATM or Frame Relay virtual-circuit identifiers, and vice versa.
Frame Relay aimed to make more efficient use of existing physical resources, which allow for the underprovisioning of data services by telecommunications companies (telcos) to their customers, as clients were unlikely to be utilizing a data service 100 percent of the time.
Telcos often sold Frame Relay to businesses looking for a cheaper alternative to dedicated lines; its use in different geographic areas depended greatly on governmental and telecommunication companies' policies.
Many customers migrated from Frame Relay to MPLS over IP or Ethernet, which in many cases reduced costs and improved manageability and performance of their wide area networks.
[35] While the underlying protocols and technologies are different, both MPLS and ATM provide a connection-oriented service for transporting data across computer networks.
Packets must be segmented, transported and re-assembled over an ATM network using an adaptation layer, which adds significant complexity and overhead to the data stream.
ATM point-to-point connections (virtual circuits), on the other hand, are bidirectional, allowing data to flow in both directions over the same path.
In practice, MPLS is mainly used to forward IP protocol data units (PDUs) and Virtual Private LAN Service (VPLS) Ethernet traffic.