Wikipedia:
Label Switch Router
A Label Switch Router (LSR) (sometimes called transit router), is a type of a router located in the middle of a Multiprotocol Label Switching (MPLS) network. It is responsible for switching the labels used to route packets.
When a LSR receives a packet, it uses the label included in the packet header as an index to determine the next hop on the Label Switched Path (LSP) and a corresponding label for the packet from a look-up table. The old label is then removed from the header and replaced with the new label before the packet is routed forward.
References
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