MPLS ConceptsOverviewThis module explains the features of Multi-protocol Label Switching (MPLS)compared to traditional ATM and hop-by-hop IP routing. MPLS concepts andterminology as well as MPLS label format and Label Switch Router (LSR)architecture and operations are explained.The module contains the following topics: Drawbacks of Traditional IP Routing Basic MPLS Concepts MPLS Labels and Label Stack MPLS Applications Differences Between Tag Switching and MPLSObjectivesUpon completion of this module, the learner will be able to perform the followingtasks: Identify the drawbacks of traditional IP routing Describe basic MPLS concepts and LSR types Describe how different MPLS applications coexist on the same platformusing the same underlying technology List the standard bodies that are working on MPLS technology and therelationship between Tag Switching and MPLS
1-2Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
Drawbacks ofTraditional IP RoutingObjectivesUpon completion of this lesson, the learner will be able to identify the drawbacksof traditional IP routing.Copyright 2002, Cisco Systems, Inc.MPLS Concepts1-3
Traditional IP Forwarding Traditional IP forwarding is based onthe following: Routing protocols are used to distributeLayer 3 (L3) routing information Forwarding is based on the destinationaddress only Routing lookups are performed on every hop 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -5Before explaining basic MPLS functionality, three drawbacks of traditional IPforwarding should be highlighted: Routing protocols are used on all devices to distribute the routinginformation. Regardless of the routing protocol, routers always forward packets based onthe destination address only. The only exception is policy-based routing(PBR) that bypasses the destination-based routing lookup. Routing lookups are performed on every router. Each router in the networkmakes an independent decision when forwarding packets.MPLS helps reduce the number of routing lookups, possibly changes theforwarding criteria, and eliminates the need to run a particular routing protocol onall the devices.1-4Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
Traditional IP inglookupRoutinglookup Destination-based routing lookup is needed on everyhop Every router may need full Internet routinginformation (more than 100,000 routes) 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -6This figure illustrates how routers in a service provider network forward packetsbased on their destination addresses. The figure also shows that all the routersneed to run a routing protocol (BGP) to get all the Internet routing information.Every router in the path performs a destination-based routing lookup in a largeforwarding table. Forwarding complexity is usually related to the size of theforwarding table and the switching mechanism.Copyright 2002, Cisco Systems, Inc.MPLS Concepts1-5
IP over 184.108.40.206.220.127.116.11.1. Layer 2 (L2) topology may be different from L3 topologyresulting in suboptimal paths and link utilization L2 devices have no knowledge of L3 routing information—virtualcircuits must be manually established Even if the two topologies overlap, the hub-and-spoke topologyis usually used because of easier management 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -7This figure illustrates a worst-case scenario where Layer 2 (L2) and Layer 3 (L3)topologies do not overlap. The result is that a single packet could be propagatedwith three L2 hops but instead requires 7 hops. The reason is that L2 devices havestatic information about how to interconnect L3 devices. Routers use a routingprotocol to propagate L3 routing information through the intermediary router.Even in another L2 topology, where the forwarding to the hub router was moreoptimal, the packet forwarding from the rightmost router to the leftmost routerwould still require unnecessary hops. The only possible solution to get optimalforwarding from any router to any other router would be to have a full mesh ofvirtual circuits. But this is rarely used due to its complexity.1-6Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
Traffic Engineering withTraditional IP ForwardingPrimaryOC-192 linkLarge site ALarge site BBackupOC-48 linkSmall site C Most traffic goes between large sites A and B and only uses theprimary link Destination-based routing does not provide any mechanism forload balancing across unequal paths Policy-based routing can be used to forward packets based onother parameters, but this is not a scalable solution 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -8This figure illustrates a topology with unequal links. Traffic patterns illustrate thatmost of the traffic goes between sites A and B.Traditional IP forwarding does not have a scalable mechanism to allow theutilization of the backup link (unequal load balancing).Policy-based routing could be used to select some packets and route those alongthe backup link. But this is not possible in high volume traffic due to performancelimitations.Copyright 2002, Cisco Systems, Inc.MPLS Concepts1-7
SummaryAfter completing this lesson, you should be able to identify the followingdrawbacks of traditional IP routing: Routing protocols with full routing information are required on all routers. Routers make a destination-based forwarding decision only. Routers must make a routing lookup on every hop.Lesson Review1. List major drawbacks of traditional IP routing.2. Based on what information do routers forward IP packets?3. What mechanism can be used to forward packets based on other parameters?4. Why is this mechanism not suitable for large networks?1-8Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
Basic MPLS ConceptsObjectivesUpon completion of this lesson, the learner will be able to perform the followingtasks: Describe MPLS architecture Describe the MPLS approach to IP routing Describe the difference between data plane and control plane in MPLS Describe the difference between packet-mode and cell-mode MPLS List LSR types Describe LSR architectureCopyright 2002, Cisco Systems, Inc.MPLS Concepts1-9
Basic MPLS Concepts MPLS is a new forwarding mechanism inwhich packets are forwarded based on labels Labels may correspond to IP destinationnetworks (equal to traditional IP forwarding) Labels can also correspond to otherparameters (QoS, source address, etc.) MPLS was designed to support forwarding ofother protocols as well 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -13MPLS is a new switching mechanism that uses labels (numbers) to forwardpackets.Labels usually correspond to L3 destination addresses (equal to destination-basedrouting). Labels can also correspond to other parameters (Quality of Service[QoS], source address, etc.).MPLS was designed to support other protocol stacks than IP as well. Labelswitching is performed regardless of the L3 protocol.1-10Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
MPLS Example10.1.1.110.1.1.1L 3Label removalandrouting lookupL 35L Routing lookupandlabel assignment10.0.0.0/8 ! L 5LabelswappingL 5 ! L 3 Only edge routers must perform a routing lookup Core routers switch packets based on simple labellookups and swap labels 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -14This figure illustrates a situation where the intermediary router does not have toperform a time-consuming routing lookup. Instead this router simply swaps alabel with another label (5 is replaced by 3) and forwards the packet based on thereceived label (3).In larger networks the result of MPLS labeling is that only the edge routersperform a routing lookup. All the core routers forward packets based on thelabels.Copyright 2002, Cisco Systems, Inc.MPLS Concepts1-11
MPLS vs. IP-over-ATM10.1.1.1L 17L 3L 510.1.1.1L2 devices run a L3 routingprotocol and establishvirtual circuits dynamicallybased on L3 information L2 devices are IP-aware and run a routing protocol There is no need to manually establish virtual circuits MPLS provides a virtual full-mesh topology 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -15This figure illustrates how MPLS is used in ATM networks to provide optimalrouting across L2 ATM switches. In order for MPLS to work with ATM switches,the switches must be somewhat L3 aware. The ATM switches must run a L3routing protocol and thus have knowledge about how to reach L3 subnets.The ATM switches automatically create a full mesh of virtual circuits based onL3 routing information. The VCs are used to forward the user data segmented intoATM cells. Another benefit of this setup is that there is no longer a need tomanually establish virtual circuits.1-12Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
Traffic Engineering with MPLSPrimaryOC-192 linkLarge site ALarge site BSecondaryOC-48 linkSmall site C Traffic can be forwarded based on other parameters(QoS, source, etc.) Load sharing across unequal paths can be achieved 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -16MPLS also supports traffic engineering. Traffic engineered tunnels can be createdbased on traffic analysis to provide load balancing across unequal paths.Multiple traffic engineering tunnels can lead to the same destination but can usedifferent paths. Traditional IP forwarding would force all traffic to use the samepath based on the destination-based forwarding decision. Traffic engineeringdetermines the path at the source based on additional parameters (availableresources and constraints in the network).Copyright 2002, Cisco Systems, Inc.MPLS Concepts1-13
MPLS Architecture MPLS has two major components: Control plane—exchanges L3 routing information andlabels Data plane—forwards packets based on labels Control plane contains complex mechanisms toexchange routing information (OSPF, EIGRP, IS-IS,BGP, etc.) and labels (Tag Distribution protocol [TDP],Label Distribution protocol [LDP], BGP, RSVP, etc.) Data plane has a simple forwarding engine Control plane maintains the contents of the labelswitching table (label forwarding information base orLFIB) 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -17To better understand the inner workings of MPLS its two major components haveto be introduced: Control plane: Takes care of the routing information exchange and the labelexchange between adjacent devices Data plane: Takes care of forwarding either based on destination addressesor labelsThere is a large number of different routing protocols such as OSPF, IGRP,EIGRP, IS-IS, RIP, BGP, etc. that can be used in the control plane.The control plane also requires protocols to exchange labels, such as: Tag Distribution Protocol [TDP] (MPLS) Label Distribution Protocol [LDP] (MPLS) BGP (MPLS virtual private networks [VPNs]) Resource-Reservation Protocol [RSVP] (MPLS Traffic Engineering [MPLSTE]) CR-LDP (MPLS-TE)The data plane however, is a simple label-based forwarding engine that isindependent of the type of routing protocol or label exchange protocol. A LabelForwarding Information Base (LFIB) is used to forward packets based on labels.The LFIB table is populated by the label exchange protocols used in the controlplane.1-14Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
MPLS Architecture (Cont.)Control planeOSPF: 10.0.0.0/8LDP: 10.0.0.0/8Label 17OSPFLDPOSPF: 10.0.0.0/8LDP: 10.0.0.0/8Label 4Data planeLabeled packetLabel 17LFIB4!!17Labeled packetLabel 4 Router’s functionality is divided into twomajor parts: control plane and data plane 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -18A simple MPLS implements destination-based forwarding that uses labels tomake forwarding decisions.A L3 routing protocol is still needed to propagate L3 routing information. A labelexchange mechanism is simply an add-on to propagate labels that are used for L3destinations.This figure illustrates the two components of the control plane: OSPF that receives IP network 10.0.0.0/8 from the left neighbor and forwardsit to the right neighbor. LDP that receives label 17 from the left neighbor to be used for packets with adestination address 10.x.x.x when forwarded to that neighbor. A local label 4is generated and sent to upstream neighbors so these neighbors can labelpackets with the appropriate label. LDP inserts an entry into Data Plane’sLFIB table where label 4 is mapped to label 17.The data plane then forwards all packets with label 4 through the appropriateinterfaces and replaces the label with label 17.Copyright 2002, Cisco Systems, Inc.MPLS Concepts1-15
MPLS Modes of Operation MPLS technology is intended to be usedanywhere regardless of Layer 1 (L1)media and L2 protocol MPLS uses a 32-bit label field which isinserted between L2 and L3 headers(frame-mode) MPLS over ATM uses the ATM headeras the label (cell-mode) 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -19MPLS is designed for use on virtually any media and L2 encapsulation. Most L2encapsulations are frame-based and MPLS simply inserts a 32-bit label betweenthe L2 and L3 headers (“frame-mode” MPLS).ATM is a special case where fixed-length cells are used and a label cannot beinserted on every cell. MPLS uses the virtual path identifier/ virtual channelidentifier (VPI/VCI) fields in the ATM header as a label (“cell-mode” MPLS).1-16Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
Label FormatLABELEXP S019 20TTL22 23 2431MPLS uses a 32-bit label field thatcontains the following information: 20-bit label3-bit experimental field1-bit bottom-of-stack indicator8-bit time-to-live field (TTL) 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -20A 32-bit label contains the following fields: 20-bit label: The actual label 3-bit experimental field: It is used to define a class of service (i.e. IPprecedence) Bottom-of-stack bit: MPLS allows multiple labels to be inserted; this bit isused to determine if this is the last label in the packet 8-bit time-to-live (TTL) field: It has the same purpose as the TTL field inthe IP headerCopyright 2002, Cisco Systems, Inc.MPLS Concepts1-17
Frame-Mode MPLSFrameheaderLayer 2IP headerPayloadLayer 3Routinglookup andlabelassignmentFrameheaderLayer 2LabelLayer 2½IP headerPayloadLayer 3 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -21This figure illustrates an edge router that receives a normal IP packet. The routerthen performs the following actions:Step 1Routing lookup to determine the outgoing interfaceStep 2Label is assigned and inserted between L2 frame header and L3 packetheader if the outgoing interface is enabled for MPLS and a next-hoplabel for the destination existsStep 3The labeled packet is sentOther routers in the core simply forward packets based on the label.1-18Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
Cell-Mode MPLSFrameheaderIP headerLayer 2FrameheaderLayer 2PayloadLayer 3LabelIP headerLayer 2½PayloadLayer 3VPI/VCI fields areused for labelswitchingCell 1ATMheaderLayer 2Cell 2ATMheader 2002, Cisco Systems, Inc.AAL5headerLabelLayer 2½IP headerPayloadLayer 3Payloadwww.cisco.comMPLS v2.1 -22Cell-mode MPLS uses the ATM header’s VPI/VCI field for forwarding decisionswhile the 32-bit label is still preserved in the frame but not used in the ATMnetwork. The original label is only present in the first cell of a packet.Copyright 2002, Cisco Systems, Inc.MPLS Concepts1-19
LSRMPLS Domain10.1.1.120.1.1.1EdgeLSRL 3L 5L 31L 4310.1.1.120.1.1.1LSR LSR primarily forwards labeled packets (labelswapping) Edge LSR primarily labels IP packets and forwardsthem into MPLS domain, or removes labels andforwards IP packets out of the MPLS domain 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -23Before proceeding with a detailed description of MPLS, some of the terminologythat is used in this course is presented: LSR: A device that primarily forwards packets based on labels Edge LSR: A device that primarily labels packets or removes labelsLSRs and Edge LSRs are usually devices that are capable of doing both labelswitching and IP routing. Their names are based on their position in an MPLSdomain. Routers that have all interfaces enabled for MPLS are called LSRsbecause they mostly forward labeled packets. Routers that have some interfacesthat are not enabled for MPLS are usually at the edge of an MPLS domain(autonomous system). These routers also forward packets based on IP destinationaddresses and label them if the outgoing interface is enabled for MPLS.1-20Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
ATM LSRMPLS Domain10.1.1.120.1.1.1L 1/3L 1/3L 1/3L 1/5L 1/5L 1/5L 1/6L 1/6L 1/6L 1/9L 1/9L 1/9ATMEdgeLSR10.1.1.120.1.1.1ATMLSR ATM LSR can only forward cells ATM Edge LSR segments packets into cells andforwards them into an MPLS ATM domain, orreassembles cells into packets and forwards themout of an MPLS ATM domain 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -24LSRs that perform cell-mode MPLS are called: ATM LSR if they are ATM switches. All interfaces are enabled for MPLSand forwarding is done of cells only, based on labels. ATM Edge LSR if they are routers connected to an MPLS-enabled ATMnetwork. Some interfaces are traditional IP interfaces where IP packets areexchanged. The IP packets are segmented into cells and forwarded into theMPLS ATM domain on cell-mode MPLS enabled interfaces. Cell traffic inthe other direction is reassembled into IP packets.Copyright 2002, Cisco Systems, Inc.MPLS Concepts1-21
Architecture of LSRs LSRs, regardless of the type, perform thefollowing three functions: Exchange routing information Exchange labels Forward packets (LSRs and edge LSRs) or cells (ATMLSRs and ATM edge LSRs) The first two functions are part of the controlplane The last function is part of the data plane 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -25LSRs of all types must perform the following functions: Exchange L3 routing information (ATM LSRs must also exchange L3 routinginformation) Exchange labels Forward packets or cellsFrame-mode and cell-mode MPLS use a different data plane: Frame-mode MPLS forwards packets based on the 32-bit label Cell-mode MPLS forwards packets based on labels encoded into the VPI/VCIfields in the ATM headerThe control plane performs the following functions:1-22 Exchange routing information regardless of the type of LSR Exchange labels according to the type of MPLS (frame-mode or cell-mode)Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
Architecture of LSRs (Cont.)LSRExchange ofrouting informationControl planeRouting protocolIP routing tableExchange oflabelsIncominglabeled packetsLabel distribution protocolData planeLabel forwarding tableOutgoinglabeled packetsLSRs primarily forward labeled packetsor cells (ATM LSRs) 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -26The primary function of an LSR is to forward labeled packets. Therefore, everyLSR needs a L3 routing protocol (OSPF, EIGRP, IS-IS, etc.) and a label exchangeprotocol (LDP, TDP, etc.).The label exchange protocol populates the LFIB table in the data plane that isused to forward labeled packets.NoteCopyright 2002, Cisco Systems, Inc.LSRs may not be able to forward unlabeled packets either because they are ATMLSRs, or they do not have all the routing information.MPLS Concepts1-23
Architecture of Edge LSRsEdge LSRExchange ofrouting informationControl planeRouting protocolIP routing tableExchange oflabelsIncomingIP packetsIncominglabeled packetsLabel distribution protocolData planeIP forwarding tableLabel forwarding tableOutgoingIP packetsOutgoinglabeled packetsNote: ATM edge LSRs can only forward cells 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -27Edge LSRs also forward IP packets based on their IP destination addresses andoptionally label them if a label exists.The following combinations are possible: A received IP packet is forwarded based on the IP destination address andsent as an IP packet A received IP packet is forwarded based on the IP destination address andsent as a labeled packet A received labeled packet is forwarded based on the label; the label ischanged and the packet is sent A received labeled packet is forwarded based on the label; the label isremoved and the packet is sent out is an IP packetThe following scenarios are possible if the network is misconfigured:1-24 A received labeled packet is dropped if the label is not found in the LFIBtable even if the IP destination exists in the FIB table A received IP packet is dropped if the destination is not found in the FIB tableeven if there is a label-switched path (LSP) available for the destinationImplementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
SummaryMPLS architecture is divided into two parts: Control plane that takes care of routing information and label propagation Data plane that takes care of the forwarding of packetsMPLS has two modes: Frame-mode MPLS that is used on all frame-based media Cell-mode MPLS that is used in MPLS-enabled ATM networksMPLS networks use the following devices: LSR to forward packets based on a 32-bit label Edge LSR to forward labeled packets or label IP packets or remove labels ATM LSRs to forward cells based on labels encoded into the VPI/VCI fieldsin the ATM header ATM Edge LSRs that segment labeled or unlabeled packets into ATM cellswhere a label is encoded into VPI/VCI fields in the ATM headerLesson Review1. What are the major drawbacks of traditional IP forwarding and how doesMPLS solve them?2. What functions does an LSR perform?3. List the types of LSRs.4. Name the two modes of MPLS.5. Explain the difference between an LSR and an Edge LSR.6. Explain the difference between an LSR and an ATM LSR.Copyright 2002, Cisco Systems, Inc.MPLS Concepts1-25
1-26Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
MPLS Labels and LabelStackObjectivesUpon completion of this lesson, the learner will be able to perform the followingtasks: Describe the format of MPLS label Explain the concept of the MPLS Label Stack Describe the way MPLS labels are used in Packet-mode and ATMenvironmentCopyright 2002, Cisco Systems, Inc.MPLS Concepts1-27
MPLS Label FormatLABELEXP S019 20TTL22 23 2431MPLS uses a 32-bit label field that contains thefollowing information: 20-bit label (a number) 3-bit experimental field (usually used to carry IPprecedence value) 1-bit bottom-of-stack indicator (indicateswhether this is the last label before the IPheader) 8-bit TTL (equal to the TTL in IP header) 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -32MPLS uses a 32-bit label that is inserted between the L2 and L3 header. AnMPLS label contains four fields:1-28 The actual label Experimental field Bottom-of-stack bit TTL fieldImplementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
MPLS Labels Labels are inserted between the L2 (frame)header and the L3 (packet) header There can be more than one label (label stack) Bottom-of-stack bit indicates if the label is thelast label in the label stack TTL field is used to prevent indefinite loopingof packets Experimental bits are usually used to carrythe IP precedence value 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -33Labels are inserted between the L2 (frame) header and the L3 (packet) header. Insome MPLS applications (e.g. MPLS VPN) more than one label is required. Inthose cases the multiple labels form label stack.Each label contains the following fields: 20-bit label: The actual label, which is a simple 20-bit number that has localsignificance and changes on every hop. 3-bit experimental field: Currently used to define a class of service such byreflecting the IP precedence of the encapsulated IP packet. Cisco routersautomatically assign the IP precedence value to this field. Bottom-of-stack bit: MPLS allows multiple labels to be inserted. Thebottom-of-stack bit is used to determine if this is the last label in the packet.This bit is set to “1” in the last label in the packet. 8-bit TTL field: It has the same purpose as the TTL field in the IP header.This field is decreased on every hop.Copyright 2002, Cisco Systems, Inc.MPLS Concepts1-29
MPLS Label StackFrameheaderLabel 1 Label 2 Label 3IP headerPayloadPID MPLS-IPS 0S 0S 1 Protocol identifier in a L2 header specifiesthat the payload starts with a label (labels)and is followed by an IP header Bottom-of-stack bit indicates whether thenext header is another label or a L3 header Receiving router uses the top label only 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -34A label does not contain any information about the L3 protocol being carried in apacket. A new protocol identifier is used for every MPLS enabled L3 protocol.This list shows the ethertype values used to identify L3 protocols with most L2encapsulations: Unlabeled IP unicast: PID 0x0800 identifies that the frame payload is an IPpacket. Labeled IP unicast: PID 0x8847 identifies that the frame payload is aunicast IP packet with at least one label preceding the IP header. The bottomof-bit indicates when the IP header actually starts. Labeled IP multicast: PID 0x8848 identifies that the frame payload is amulticast IP packet with at least one label preceding the IP header. Thebottom-of-bit indicates when the IP header actually starts.A router that receives a frame where the PID indicates that it is a labeled packetuses only the top label in stack for forwarding decisions.1-30Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
MPLS Label Stack (Cont.) Usually there is only one label assigned to a packet The following scenarios may produce more than onelabel: MPLS VPNs (two labels—the top label points to the egressrouters and the second label identifies the VPN) Traffic Engineering (MPLS-TE) (two or more labels—thetop label points to the endpoint of the traffic engineeringtunnel and the second label points to the destination) Any Transport over MPLS (two labels—the top label pointsto the egress routers and the second label identifies theoutgoing interface) MPLS VPNs combined with Traffic Engineering (three ormore labels) 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -35As previously noted, MPLS supports multiple labels in one packet. Simple MPLSuses just one label in each packet. The following applications may add additionallabels to packets: MPLS VPNs use multiprotocol BGP to propagate a second label that is usedin addition to the one propagated by TDP or LDP. MPLS-TE uses RSVP to establish label-switched tunnels. RSVP alsopropagates labels that are used in addition to the one propagated by LDP orTDP. Any Transport over MPLS (AToM) uses a directed multihop LDP sessionbetween the edge routers to propagate a second label that is used in additionto the one propagated by the per link LDP- or TDP-sessions. A combination of the above mentioned mechanisms with some other featuresmight result in three or more labels being inserted into one packet.Copyright 2002, Cisco Systems, Inc.MPLS Concepts1-31
MPLS Forwarding An LSR can perform the followingfunctions: Insert (impose) a label or a stack of labels oningress. Swap a label with a next-hop label or a stackof labels in the core. Remove (pop) a label on egress. ATM LSRs can only swap a label withone label (VPI/VCI fields change) 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -36An IP packet going through an MPLS domain experiences the following: A label or a stack of labels is inserted (imposed) on an Edge LSR The top label is swapped with a next-hop label or a stack of labels on an LSR The top label is removed on the LSP endpoint (usually one hop before theegress Edge LSR or on the egress edge LSR itself)ATM LSRs only support the swapping of one label (normal ATM operation).1-32Implementing Cisco MPLS (MPLS) v2.1Copyright 2002, Cisco Systems, Inc.
MPLS Forwarding(Frame-Mode)MPLS Domain10.1.1.1310.1.1.1510.1.1.110.1.1.1IP Lookup10.0.0.0/8 ! label 3IP Lookup10.0.0.0/8 ! label 5IP Lookup10.0.0.0/8 ! next hopLFIBlabel 8 ! label 3LFIBlabel 3 ! label 5LFIBlabel 5 ! pop On ingress a label is assigned and imposed by the IP routing process LSRs in the core swap labels based on the contents of the labelforwarding table On egress the label is removed and a routing lookup is used to forwardthe packet 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -37This figure illustrates an MPLS network using frame-mode MPLS. All LSRs arecapable of forwarding IP packets or labeled packets. The ingress edge LSRperforms a routing lookup and assigns a label. The middle router simply swapsthe label. The egress LSR removes the label (penultimate hop popping is coveredlater) and optionally performs a routing lookup.Copyright 2002, Cisco Systems, Inc.MPLS Concepts1-33
MPLS Forwarding(Cell-Mode)MPLS Domain10.1.1.11/3 1/3 1/3 1/31/5 1/5 1/5 1/510.1.1.1IP Lookup10.0.0.0/8 ! label 1/3IP Lookup10.0.0.0/8 ! label 1/5IP Lookup10.0.0.0/8 ! Next hopLFIBlabel 8 ! label 1/3LFIBlabel 1/3 ! label 1/5LFIBlabel 1/5 ! pop Labels (VPI/VCI) are imposed during the IP lookup process on ingress ATM edgeLSRs. Packets are segmented into cells. ATM LSRs in the core swap labels based on the contents of the ATM switchingtable. ATM LSRs cannot forward IP packets. On egress ATM edge LSRs the labels are removed (cells are reassembled intopackets) and a routing lookup is used to forward packets. 2002, Cisco Systems, Inc.www.cisco.comMPLS v2.1 -38Cell-mode MPLS is similar to frame-mode MPLS. The difference is that ATMLSRs (ATM switches) are not capable of forwarding IP packets:The ingress ATM edge LSR (router) performs an IP routing table lookup andfinds the outgoing interfac
MPLS Concepts Overview This module explains the features of Multi-protocol Label Switching (MPLS) . Each router in the network . MPLS provides a virtual full-mesh topology L 5 10.1.1.1 L 3 10.1.1.1 L 17 L2 devices run a L3 routingFile Size: 300KB