A label switch router (sometimes called transit router), is a type of a router located in the middle of a Multiprotocol Label Switching

Multiprotocol Label Switching

Multiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...

 (MPLS) network. It is responsible for switching the labels used to route packets.

When an 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

Label Switched Path

In MPLS networking, a Label Switched Path is a path through an MPLS network, set up by a signaling protocol such as LDP, RSVP-TE, BGP or CR-LDP. The path is set up based on criteria in the forwarding equivalence class ....

 (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.