Label switch router

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A label switch router (LSR) is the device that sits within an Multiprotocol Label Switching (MPLS) network. That is to say, all of its interfaces connect it to other LSRs, or to label edge routers (LER). When the MPLS network is deployed by a common carrier, it is also known as the provider (P) router.

The LSR receives a transmission unit with an MPLS label (or tag) affixed and examines the label which identifies a specific label switched path (LSP) along which the transmission unit is to flow. The LSR typically swaps the incoming label with the appropriate outgoing label which the router has looked up in its forwarding information base (FIB). The LSR will then queue the transmission unit for transmission on the appropriate output port. The LSR uses the value of the EXP field within the label to determine specifically which queue and how to handle the transmission unit. The EXP field is used as a class of service identifier these days. If the LSR is the penultimate router along the LSP, it may be responsible for popping the label before sending it on to the LER for appropriate delivery outside the MPLS network. This penultimate hop popping is under the control of the LER when the LSP was first being established. The purpose is to alleviate some of the work for the LER which will be terminating many LSPs and will save the LER double lookups of the label then the address of the transmission unit below the label. In some instances penultimate hop popping is not possible due to the underlying network being able to carry the raw transmission unit.

The LSR play several other roles as well. It participates in label distribution using a label distribution protocol (LDP). It participates in fast reroute to recover from failures within the MPLS network. In general, it engages in many activities related to traffic engineering (TE).


<mp3></mp3> | Label edge router vs. label switch router (LSR)