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Multiprotocol label switching
Multiprotocol Label Switching

It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients which provide a datagram service model. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames. MPLS operates at an OSI Model layer that is generally considered to lie between traditional definitions of Layer 2 (data link layer) and Layer 3 (network layer), and thus is often referred to as a "Layer 2.5" protocol.

Goals of mpls
Goals of MPLS

  • Scalability of network layer routing.

    • Using labels as a means to aggregate forwarding information,while working in the presence of routing hierarchies.

  • Greater flexibility in delivering routing services.

    • Using labels to identify particular traffic which are to receive special services, e.g. QoS.

  • Increased performance.

    • Using the label-swapping paradigm to optimize network performance.

Goals of mpls1
Goals of MPLS

  • Simplify integration of routers with cell switching based technologies.

    • Making cell switches behave as routers.

    • By making information about physical topology available to network layer routing procedures.

Mpls qos

  • MPLS improves internet scalability by eliminating the need for each router and switch in a packet's path to perform traditionally redundant address lookups and route calculation.

    • Improves scalability through better traffic engineering.

  • MPLS also permits explicit backbone routing, which specifies in advance the hops that a packet will take across the network.

  • This should allow more predictable, performance that can be used to guarantee QoS.

Introduction to mpls
Introduction to MPLS

  • The paths function at layer 3 can even be mapped directly to layer 2 transport such as ATM or frame relay.

  • Explicit routing will give IP traffic a semblance of end-to-end connections over the backbone.

  • The MPLS definition of IP QoS parameters is limited.

  • Out of 32 bits total, an MPLS label reserves just three bits for specifying QoS.

Introduction to mpls1
Introduction to MPLS

  • Label-switching routers (LSRs) will examine these bits and forward packets over paths that provide the appropriate QoS levels. But the exact values and functions of these so-called 'experimental bits‘ remain to be defined.

  • The MPLS label could specify whether traffic requires constant bit rate (CBR) or variable bit rate (VBR) service, and the ATM network will ensure that guarantees are met.

Mpls architecture
MPLS Architecture

MPLS Egress Node

MPLS Ingress Node


  • A label is short, fixed length physically continuous identifier which is used to identify a FEC ( forwarding equivalence class), usually of local significance.

  • Ru can transmits a packet labeled L to Rd, if they can agree to a binding between label L and FEC F for packets moving from Ru to Rd.

    • Ru (upstream LSR)  Rd (downstream LSR with respect to a given binding).

    • L becomes Ru’s “outgoing label” representing FEC F, and L becomes rd’s “incoming label” representing FEC F.

    • Rd must make sure that the binding from label to FEC is one-to-one.


  • Rd must not agree with Ru1 to bind L to FEC F1, while agreeing with some other LSR Ru2 to bind L to a different FEC F2, unless rd can always tell, when it receives a packet with incoming label L, whether the label was put on the packet by Ru1 or Ru2.


L for FEC F1



L for FEC F2

Labeled packet
Labeled Packet

  • A packet into which a label has been encoded.

  • The label resides in an encapsulation header which exists specifically for this purpose.

  • Or the label may reside in a existing data link or network layer header.

  • The particular encoding technique which is used must be agreed to by both the entities which encodes the label and the entity which decodes the label.

Label assignment and distribution
Label Assignment and Distribution

  • The decision to bind a particular label L to a particular FEC F is made by the LSR which is downstream with respect to that binding.

  • The downstream LSR informs the upstream LSR of the binding.

  • The labels are ‘downstream assigned’ and label binding are distributed in the ‘downstream to upstream’ direction.

Label distribution protocols
Label Distribution Protocols

  • It is set of procedures by which one LSR informs another LSRs of the bindings (label/FEC) it has made.

  • Two LSRs which use a distribution protocol to exchange label/FEC binding information are known as “label distributing peers” with respect to the binding information they exchange.

  • There exists many different distribution protocols ( [MPLS- BGP], [MPLS-RSVP], [MPLS-RVSP-TUNNELS], [MPLS-CR-LDP]).

Unsolicited downstream vs downstream on demand
Unsolicited Downstream Vs. Downstream-on-demand

  • Downstream-on-demand label distribution.

    • An LSR explicitly request (a label binding for that FEC ),from its next hop for a particular FEC.

  • Unsolicited downstream label distribution.

    • LSR distribute bindings to LSRs that have not explicitly requested them.

  • Both these label distribution techniques can be used in the same network at the same time.

  • Which protocol is provided by the MPLS implementation depends on the characteristics of the interfaces which are supported by a particular implementation.

Label retention mode
Label Retention Mode

  • An LSR Ru receives a label binding for a particular FEC from an LSR Rd, even though Rd is not Ru’s next hop. Ru then has the choice of whether to keep track or discard it.

  • Liberal label retention mode.

    • It maintains the bindings.

    • Allows for quicker adaptation to routing changes.

  • Conservative label retention mode.

    • It discards such bindings.

    • Requires an LSR to maintain few labels.

The label stack
The Label Stack

  • Label stack carries a number of labels organized as a last-in, first out stack.

  • The processing is always based on the top label.

  • An unlabeled packet can be thought as a packet whose label stack is empty.






  • NHLFE (Next Hop Label Forwarding Entry) is used when forwarding a labeled packet.

  • It contains the following information.

    • The packet’s next hop.

    • The operation to perform on the packet’s label stack.

      • Replace the label at the top of the label stack with a specified new label.

      • Pop the label stack.

      • Replace the label at the top of the label stack with a specified new label, and then push one or more specified new labels onto the label stack.

Incoming label map ilm
Incoming Label Map (ILM)

  • Maps each incoming label to a set of NHLFEs.

  • Used when forwarding packets that arrive as labeled packets.

  • Exactly one element of set must be chosen before the packet is forwarded.

    • It is used to load balance over multiple equal-cost paths.

Set of NHFLE


Fec to nhfle map ftn

  • FTN maps each FEC to a set of NHFLEs.

  • It is used when forwarding packets that arrive unlabeled, but are labeled before being forwarded.

Set of NHFLE


Label swapping
Label Swapping

  • In order to forward a labeled packet, a LSR examines the label at the top of the label stack. It uses the ILM to map this label to an NHLFE.

  • Using the information in the NHFLE, it determines where to forward the packet, and performs an operation on the packet’s label stack. It then encodes the new label stack into the packet, and forwards the result.

Label swapping1
Label Swapping

  • In order to forward an unlabeled packet, a LSR analyzes the network layer header, to determine the packet’s FEC. It then uses FTN to map this label to an NHFLE.

  • Using the information in the NHFLE, it determines where to forward the packet, and performs an operation on the packet’s label stack. It then encodes the new label stack into the packet, and forwards the result.