MPLS which stands for 'Multi-Protocol Label Switching ' evolved from a number of vendor projects in the mid-1990s that were aimed at making IP routing more efficient and more competitive with protocols like frame relay and ATM. The primary source of MPLS standards was Cisco Systems' 'tag switching ' a mechanism for creating what were in effect specific routes for traffic through IP networks. That remains the basis for the core of MPLS today.
In routed IP networks traffic moves between routers based on adaptive topology updates based on the exchange of 'reachability' information among network routers. Traffic was routed 'hop by hop' from router to router following a path that could be determined only by examining the sum of the routing tables in the network at the time the traffic was moving. This made it impossible to engineer specific quality of service by allocating resources to traffic types. The goal of MPLS was to create special entries in routing tables that combined to create label-switched paths (LSPs) or multi-hop routes.
MPLS LSPs are normally created using the IP routing tools. MPLS like IP has automatic adaptive routing but MPLS LSPs can be threaded among devices and over paths in a way that supports traffic engineering. So it has become an almost-universal way of adding quality-of-service or application-specific routing support to IP networks.
The adaptive nature of MPLS routing means that it recovers automatically from network device or link failures and so in the late 1990s work was begun to utilize MPLS principles to route optical or circuit-switched traffic. This involved adding an IP/MPLS control plane as a layer above the optical/TDM switching network and allowing connections to be routed via IP/MPLS principles first then 'pushed down' to the data plane. This resulted in what was called Generalized MPLS or GMPLS and it is now common to read about 'GMPLS control planes' as a means of finding a path between two points using IP/MPLS principles even if the network isn't an IP network.
The problem with MPLS according to some network operators is that it's a protocol over IP. First this means it must be implemented on relatively expensive IP routers. Second it is still subject to adaptive automatic rerouting in the event of failures which can still create problems with QoS control and interfere with service provider policies on failover traffic handling. In short the problems that MPLS and IP have make them less than perfect in supporting at least a class of network applications.
MPLS alternative to Carrier Ethernet's PBT
One solution to the MPLS challenges was to solve the problems at the Carrier Ethernet level by enhancing Carrier Ethernet's Provider Backbone Bridging (PBB) standard to support what were essentially Ethernet multi-hop paths. This created the Provider Backbone Transport (PBT) or PBB-TE (PBB with Traffic Engineering) standard currently being finalized. PBT eliminates the bridging discovery protocols of Ethernet relying instead on a separate control plane to maintain the bridging tables that control traffic flow. One logical protocol to use for this is GMPLS though any standard or proprietary mechanism to find traffic paths would be workable.
PBT created a considerable flap in the IP world where it was seen as a threat to IP convergence and many IP vendors are totally opposed to it.