MPLS is a multi-vendor, multiple protocols standard to enable very fast packet switching at the central core of a network.

Packets are tagged as they enter an MPLS switching environment with a Forwarding Equivalence Class (FEC).

At a very oversimplified level, an FEC label is like a zipcode. At the main switching core of a network, the packets do not need to be routed based on individual street addreses and cubile numbers - only to a specific region of the country. The mail sorting and delivery system does not need to know what is written on the pages inside the envelope.

However, the sender of the message may need priority delivery of that envelope - in which case the sender is willing to pay extra to FedEx or Priority mail to get special treatment. Voice over IP in MPLS (for example) could be flagged as getting special treatment in our example.

Lower priority traffic (junk mail, catalogs) can be delayed or discarded if congestion develops. Individual VPNs are kept safely away from each other's traffic while using shared resources and accessing shared network services.

Labels can be nested so that the packet can be routed through mutiple layers of Switch Routers effectively.

Because MPLS is not protocol specific, MPLS can route ATM, Frame Relay, Ethernet, Optical and future OSI compliant protocols - since MPLS looks at the tags to make its routing decisions, not Level 3 headers. MPLS is wedged into the headers between level 2 and level 3 of the OSI model. (level 2.5?)

A non-MPLS network hands its traffic to the MPLS cloud at a Label Edge Router (LER). The LER configuration identifies the appropriate delivery label(s) needed and MPLS does the rest. Label Switch Routers determine the best route based on the label(s) and the traffic arrives at the other end of the MPLS cloud at the appropriate destination LER. To the typical end-user, they are not aware their packets traversed an MPLS cloud using shared bandwidth. MPLS also transparently routes around failed paths or congestion in the MPLS network cloud.

For the telecommunications service provider, MPLS greatly simplifies creation and management of VPN networks and VPN access to shared resources. Technologies like Multicast are much more efficient and easier to implement with MPLS, especially if the multicast traffic is delivered over VPNs (think stock tickers devlivered to a subscription based service over a VPN, as an example).