Multiprotocol Label Switching (MPLS) is data forwarding technology that increases the speed and controls the flow of network traffic. With MPLS, data is directed through a path via labels instead of requiring complex lookups in a routing table at every stop. It is a mechanism used to transfer data across large data/voice/video networks.

Each packet entering an MPLS network is labeled with a locally significant MPLS label. As the packet passes through the MPLS network, label is replaced with another label or stripped off. The network distributes information so that each switch knows what it is supposed to do if it encounters a particular label by simply looking up the MPLS table. Thus, the router functionality is reduced to switch functionality, speeding up the data transfer significantly.

MPLS is a scalable and protocol-independent routing technique. It works with Internet Protocol (IP), Ethernet, Frame Relay and Asynchronous Transport Mode (ATM). Despite the advent of newer technologies, it remains relevant due to its features like security, flexibility and traffic engineering.

In an MPLS network, the first router to receive a packet determines the packet's entire route upfront, the identity of which is quickly conveyed to subsequent routers using a label in the packet header.

Working Principle Of MPLS: MPLS works by prefixing 32-bit labels with the MPLS header. The 32-bit label contains four fields −

• Label value field of 20-bits
• Traffic class field of 3-bits for QoS (quality of service)
• Bottom of stack flag of 1-bit (1 value denotes that the current label is the last one in the stack).
• TTL (time to live) field of 8-bits.

When an IP packet enters the MPLS network, the 32-bit MPLS label is added by the ingress router, which is a label edge router (LER). LER decides the virtual path called label-switched path (LSP) that the packet will follow until it reaches its destination. The subsequent label-switching routers (LSRs) along the LSP, forwards the packet based upon only the MPLS labels. They do not look beyond the MPLS label to the IP header. When the packet reaches the egress router (also an LER), the MPLS labels are removed and the original IP packet is forwarded towards the final destination.