MPLS: how it works and who should use it

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 MPLS: how it works and who should use it

Written by: Les Harper - Feb 9, 2010

MPLS: how it works and who should use it

MPLS is an abbreviation for Multiprotocol Label Switching (MPLS). It belongs to a family of packet switched networks. This mechanism is used in telecommunications networks to direct and carry data from one network node (computer/telephone) to another. This creates virtual links between distant nodes. It is scalable and protocol agnostic and hence it can encapsulate packets (pieces of information) from various network protocols and carry them across the network.

Each packet pushed into the network for transmission is assigned a label. The label is read without accessing the contents of the packet and transmitted to the correct destination without reference to any protocol. Hence there is no dependence on any particular technology. It can, therefore, replace a number of technologies that were previously deployed for data transmission and, is fast doing so.

On the flip side of this technology is the fact that MPLS can result in a significant loss of visibility and access into the MPLS could for deploying IT departments.

The initial concept of MPLS technology was first developed by engineers from Ipsilon Networks. Cisco did similar work in the development of “Tag Switching” and this was handed over to IETF for open standardization. A number of vendors were then involved in the final design of the MPLS technology. This is the reason why this technology is vendor independent; permits the creation of simple high speed switches and supports multiple service models for traffic management. The robust recovery framework goes beyond simple protection rings that are available with synchronous optical networking protocols.

The labels that are assigned to the different packets are called MPLS headers. A set of packets containing MPLS headers is known as a stack. Each label contains four fields--the label value; the 3 bit traffic class or priority; 1 bit bottom of the stack flag and 8 bit time to live field. The entry and exit points of the network are called Label edge routers. These label routers push the MPLS label onto the incoming packet and pop it off the outgoing packet. The routers then exchange label reachability information with each other using standardized procedures and build a complete picture of the network for forwarding packets. At the end of the network, the labels are examined once again and based on the contents the packets are swapped, pushed or popped. Pre-built look up tables tell the MPLS what kind of operation needs to be performed on the basis of the topmost label of the incoming packet.

MPLS is often installed with two types of standardized protocols—the CR-LDP or Constraint based Routing Label distribution protocol and RSVP-TE or Resource Reservation Protocol. The MPLS works in conjunction with the IP or Internet Protocol.

MPLS is currently used only with “IP only” networks. MPLS also allows for creation of Virtual Private Networks for providing traffic isolation for those who want a secure system in place. MPLS can also be used for Martini Drafts that define how MPLS can be used to support Layer 2 transport services such as Ethernet, Frame Relay or ATM. These drafts also define the encapsulation methods in Layer 2 an the transport signalling methods of this layer. Many other service providers use MPLS for providing easy migration to L2 service customers. A number of other services are also provided over MPLS networks with a range of choices for the convenience of customers.