Label Switching and Label-switched Paths (LSPs) in MPLS
Before the transmission of packets can take place between a specific source-destination, the path between the source-destination through which the packet needs to pass needs to be determined. That trajectory called LSPs (label-switched paths). Routers in an MPLS network exchange information (routing information, etc.) to create these LSP routes for different source-destination pairs. All routers on a common LSP path will have the same understanding of this LSP path.
In MPLS, data packets not transmitted on a hop-by-hop basis as in an IP network, ie data transmitted over a pre-defined path between source and destination and the number plate attached to that data will be switched (label switched) by the routers on the LSP path where the data must pass through. These routers called LSR (label switch routers). The LSR will be change the label number between the input (IN) for the data receiver and the output (OUT) for its data transmission. Meaning that the input data with an in-label will be converted to a number plate Out label (Attach data to be forwarded with out label) before data is forwarded based on label mapping table or label forwarding table) of it. Label switching will be perform by various LSRs on the LSP pathway, including the LSR egress.
An LSP path starts from the first node, called the Label Edge Router (LER) or ingress LSR, where the label packaged with the data packet and placed on an LSP path forever. To the last node, called the LER (Label Edge Router) or egress LSR, where the label will be remove from the data packet to forward to the final destination.