MPLS Terms and Acronyms

The following table defines common MPLS (Multiprotocol Label Switching) terms and acronyms.

Click to expand in new window

MPLS Terms and Acronyms

Term or Acronym Description


Constrained Shortest Path First. Route selection determined by an algorithm based on available link bandwidth and path cost.


Downstream-on-Demand. Distribution of labels as a result of explicit upstream label requests.


Downstream Unsolicited. Distribution of labels downstream without an explicit label request.

EXP bits

A three-bit experimental field in an MPLS shim header.


Forward Equivalence Class. A group of packets that are forwarded in the same manner (for example, over the same Label Switched Path).


A short, fixed-length identifier used to forward packets from a given link.

Label stack

A set of one or more MPLS labels used by MPLS to forward packets to the appropriate destination.

Label swapping

Lookup and replacement of an incoming label with the appropriate outgoing label.


Label Distribution Protocol. A protocol defined by the IETF used to establish an MPLS Label Switched Path (LSP).


Label Edge Router. A Label Switch Router that is at the beginning (ingress) or end (egress) of an LSP.


Label Switched Path. The unidirectional MPLS connection between two routers over which packets are sent. LSPs are established using LDP or RSVP-TE.


Label Switch Router. A router that receives and transmits packets on an MPLS network.


MultiProtocol Label Switching. A set of protocols defined by the IETF used to transmit information based on a label-switching forwarding algorithm.


Next Hop Label Forwarding Entry. The NHLFE represents the MPLS router next hop along the LSP.


Penultimate Hop Popping. A label stack optimization used for conserving the number of allocated labels.


Pseudowire. A logical point-to-point connection.


Resource ReSerVation Protocol (RSVP). A resource setup protocol designed for an integrated services network.


Resource ReSerVation Protocol-Traffic Engineering (RSVP-TE). The combination of RSVP and MPLS label signaling to provide traffic engineered LSPs as specified in RFC 3209, RSVP-TE: Extensions to RSVP for LSP Tunnels.

Shim header

MPLS-specific header information that is inserted between layer-2 and layer-3 information in the data packet.


Service Provider. An entity that provides network services for individuals or organizations.


Traffic Engineering. The provisioning of an autonomous flow along a specified network path.

Transport LSP

Any active LSP used to forward traffic through an MPLS network.


Virtual Private LAN Service (VPLS). A multipoint Layer 2 VPN service that has the property that all PW tunnels within a VPN are signaled with the same vcid, where the vcid represents the VPN identifier.


Virtual Private Network (VPN). A logical private network domain that spans a public or service provider network infrastructure.


Virtual Private Wire Service (VPWS). A point-to-point Layer 2 VPN service that operates over MPLS.