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MPLS ArchitectureOverviewJay Kumarasamyjayk@cisco.comAdopted from Stefano Previdi’s presentation 2001, Cisco Systems.1Agenda MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths LDP overview Day in the Life of a Packet2001 Cisco Systems, Inc.www.cisco.com2
MPLS Concepts MPLS: Multi Protocol Label Switching MPLS is a layer 2 switching Developed to integrate IP and ATM MPLS forwarding is done in the same wayas in ATM switches Packet forwarding is done based onLabels2001 Cisco Systems, Inc.www.cisco.com3MPLS Concepts Unlike IP, classification/label can be basedon:Destination Unicast addressTraffic EngineeringVPNQoS FEC: Forwarding Equivalence Class A FEC can represent a: Destination addressprefix, VPN, Traffic Engineering tunnel, Classof Service.2001 Cisco Systems, Inc.www.cisco.com4
Agenda MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths LDP overview Summary2001 Cisco Systems, Inc.www.cisco.com5LSRs and Labels LSR: Label Switch Router Edge-LSR: LSRs that do label impositionand disposition2001 Cisco Systems, Inc.www.cisco.com6
LSRs and LabelsIGP domain with a labeldistribution protocol An IP routing protocol is used within the routing domain(e.g.:OSPF, i-ISIS) A label distribution protocol is used to distribute address/labelmappings between adjacent neighbors The ingress LSR receives IP packets, performs packetclassification, assign a label, and forward the labelled packet intothe MPLS network Core LSRs switch packets/cells based on the label value The egress LSR removes the label before forwarding theIP packet outside the MPLS network2001 Cisco Systems, Inc.www.cisco.com7LSRs and Labels012301234567890123456789012345678901Label Exp S TTLLabel 20 bitsExp Experimental, 3 bitsS Bottom of stack, 1bitTTL Time to live, 8 bits More than one Label is allowed - LabelStack MPLS LSRs always forward packets based onthe value of the label at the top of the stack2001 Cisco Systems, Inc.www.cisco.com8
LSRs and LabelsPPP Header(Packet overSONET/SDH)EthernetFrame RelayATM Cell Header GFCPPP HeaderShim HeaderLayer 3 HeaderEthernet HdrShim HeaderLayer 3 HeaderFR HdrShim HeaderLayer 3 HeaderVPIVCIPTI CLP HECDATAVCIPTI CLP HECDATALabelSubsequent cells GFCVPILabel2001 Cisco Systems, Inc.www.cisco.com9Agenda2001 Cisco Systems, Inc. MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths LDP overview Day in the Life of a Packetwww.cisco.com10
Label Assignment andDistribution Labels have link-local significanceEach LSR binds his own label mappings Each LSR assign labels to his FECs Labels are assigned and exchanged betweenadjacent neighboring LSR Applications may require non-adjacentneighbors2001 Cisco Systems, Inc.www.cisco.com11Label Assignment andDistributionUpstream and Downstream LSRs171.68.40/24171.68.10/24Rtr-ARtr-BRtr-C Rtr-C is the downstream neighbor of Rtr-B for destination171.68.10/24 Rtr-B is the downstream neighbor of Rtr-A for destination171.68.10/24 LSRs know their downstream neighbors through the IProuting protocolNext-hop address is the downstream neighbor2001 Cisco Systems, Inc.www.cisco.com12
Label Assignment andDistributionUnsolicited Downstream DistributionUse label 30 for destination171.68.10/24Use label 40 for /FInLab0-171.68.10130.030 171.68.10. .NextNext-Hop. LabAddressPrefix040 171.68.10OutI/FOutLab1.NextNext-Hop. .IGP derived routes LSRs distribute labels to the upstream neighbors2001 Cisco Systems, Inc.www.cisco.com13Label Assignment andDistributionOn-Demand Downstream DistributionUse label 40 for destination171.68.10/24Use label 30 for destination171.68.10/24171.68.10/24171.68.40/24 Rtr-ARtr-BRequest label fordestination 171.68.10/24Rtr-CRequest label fordestination 171.68.10/24 Upstream LSRs request labels to downstream neighbors Downstream LSRs distribute labels upon request2001 Cisco Systems, Inc.www.cisco.com14
Label Assignment andDistributionSeveral protocols for label exchange LDPMaps unicast IP destinations into labels RSVP, CR-LDPUsed in traffic engineering BGPExternal labels (VPN) PIMFor multicast states label mapping2001 Cisco Systems, Inc.www.cisco.com15Agenda2001 Cisco Systems, Inc. MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths LDP overview Day in the Life of a Packetwww.cisco.com16
Label Switch Path (LSP)IGP domain with a labeldistribution protocolIGP domain with a labeldistribution protocolLSP follows IGP shortest pathLSP diverges from IGP shortest path LSPs are derived from IGP routing information LSPs may diverge from IGP shortest pathLSP tunnels (explicit routing) with TE LSPs are unidirectionalReturn traffic takes another LSP2001 Cisco Systems, Inc.www.cisco.com17Label Switch Path (LSP)Penultimate Hop Popping The label at the top of the stack is removed(popped) by the upstream neighbor of theegress LSR The egress LSR requests the “popping”through the label distribution protocol Egress LSR advertises implicit-null label The egress LSR will not have to do a lookupand remove itself the label One lookup is saved in the egress LSR2001 Cisco Systems, Inc.www.cisco.com18
Label Switch Path (LSP)Penultimate Hop PoppingInI/F0InLab-.AddressPrefix171.68/16Out OutI/F Lab14NextNext. tNext. -Hop.Summary routefor 171.68/1601AddressNextNext-HopPrefix and ary routefor 171.68/160171.68.44/24Use label 4 forFEC 171.68/16Use label “implicit-null”for FEC 171.68/16171.68.10/24Egress LSR summarises morespecific routes and advertisesa label for the new FECSummary route is propagate throughthe IGP and label is assigned by eachLSREgress LSR needs to do an IP lookup for finding morespecific routeEgress LSR need NOT receive a labelled packet2001 Cisco Systems, Inc.www.cisco.com19Agenda2001 Cisco Systems, Inc. MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths LDP overview Day in the Life of a Packetwww.cisco.com20
LDP Concepts Label Distribution Protocol Labels map to FECs for Unicast DestinationPrefix LDP works between adjacent/non-adjacentpeers LDP sessions are established between peers2001 Cisco Systems, Inc.www.cisco.com21LDP Messages Discovery messages Used to discover and maintain the presence ofnew peers Hello packets (UDP) sent to all-routers multicastaddress Once neighbor is discovered, the LDP session isestablished over TCP2001 Cisco Systems, Inc.www.cisco.com22
LDP Messages Session messages Establish, maintain and terminate LDP sessions Advertisement messages Create, modify, delete label mappings Notification messages Error signalling2001 Cisco Systems, Inc.www.cisco.com23Agenda MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths ATM LSRs Loops and TTL LDP overview Day in the Life of a Packet2001 Cisco Systems, Inc.www.cisco.com24
Day in the life of a PacketInI/F0.InLab.AddressPrefix171.68/16Out OutI/F Lab14NextNext. -Hop.InI/F0InLab4.AddressPrefix171.68/16Out OutI/F Lab17NextNext. -Hop.InI/F0.P11PEUse label 4 forFEC 171.68/16AddressPrefix171.68/16OutI/F2NextNext. -Hop.OutLabpop.AddressNextNext-HopPrefix and 4/24171.68.12.1Serial2171.68/16.Null2PE00Use label “implicit-null”for FEC 171.68/16Use label 7 forFEC 171.68/16171.68.44/24Summary routefor 171.68/16Summary routefor 171.68/16CE.P00InLab7171.68.10/24Summary route is propagate throughthe IGP and label is assigned by eachLSREgress LSR summarises morespecific routes and advertisesa label for the new FECEgress LSR needs to do an IP lookup for finding more specific route2001 Cisco Systems, Inc.www.cisco.com25Summary LSRs forward packet based on labelinformation IP header and forwarding decision havebeen de-coupled for better flexibility Label information can derive from differentsourcesIP routing protocols (destination based unicast routing)MulticastTraffic EngineeringQoSVPN2001 Cisco Systems, Inc.www.cisco.com26
Summary MPLS allows flexible packet classificationand network resources optimisation Labels are distributed by differentprotocols LDP, RSVP, BGP, PIM Different distribution protocols may coexist in the same LSR Label have local (LSR) significanceNo need for global (domain) wide labelallocation/numbering2001 Cisco Systems, Inc.www.cisco.com27Thank You!2001 Cisco Systems, Inc.www.cisco.com28
3 2001 Cisco Systems, Inc. www.cisco.com MPLS Concepts MPLS: Multi Protocol Label Switching MPLS is a layer 2 switching Developed to integrate IP and ATM MPLS forwarding is done in the same way as in ATM switches Packet forwarding is done based on Labels 4 2001 Cisco Systems, Inc. www.cisco.com MPLS Concepts Unlike IP, classification/label can be based
