Introduction to Segment Routing

1 Introduction to Segment RoutingSegmentRouting(SR) is a flexible,scalableway of doingsourcerouting. Overviewof SegmentRouting,page1 How SegmentRoutingWorks,page2 Examplesfor SegmentRouting,page3 Benefitsof SegmentRouting,page4 SegmentRoutingGlobalBlock,page6 AdditionalReferencesfor SegmentRouting,page8 Overview of Segment RoutingSegmentRouting(SR) is a flexible,scalableway of sourcechoosesa path andencodesit in the packetheaderas an orderedlist of identifierfor any type of segmentis identifiedby the segmentID (SID)consistingof a flat unsigned32-bitinteger. Segmentinstructioncan be: Go to node N usingthe shortestpath Go to node N over the shortestpath to node M and then followlinks Layer1, Layer2, and Layer3 ApplyserviceSWith segmentrouting,the networkno longerneedsto maintaina per-applicationand ,it obeysthe forwardinginstructionsprovidedin the a smallnumberof extensionsto CiscoIntermediateSystem-to-IntermediateS ystem(IS-IS)and OpenShortestPath First (OSPF) can operatewith an MPLS(MultiprotocolLabelSwitching)or an IPv6 data plane,and it integrateswith the rich multiservicecapabilitiesof MPLS,includingLayer3 VPN (L3 VPN),VirtualPrivateWire Service(VPWS),VirtualPrivateLANS ervice(VPLS),andEthernetVPN (EVPN).

2 Segmentroutingcan be directlyappliedto the MultiprotocolLabelSwitching(MPLS)archite cturewith nochangein the networkbandwidthmoreeffectivelythantradi tionalMPLS networksand offers lowerlatency. A segmentis encodedas an orderedSegment Routing Configuration Guide, Cisco IOS XE Release 3S 1list of segmentsis encodedas a stackof segmentto processis on the top of the relatedlabel is poppedfrom the stack,after the completionof a be appliedto the IPv6 architecturewith a new type of routingextensionheader. Asegmentis encodedas an IPv6 orderedlist of segmentsis encodedas an orderedlist of IPv6addressesin the routingextensionheader. The segmentto processis indicatedby a pointerin the routingextensionheader. The pointeris incremented,after the completionof a protectionwithoutany networkprotectstraffic againstlink and node failureswithoutrequiringadditionalsignal ingin the fast re-route(FRR)technology, in combinationwith the explicitroutingcapabilitiesin SegmentRoutingguaranteesfull protectioncoveragewith protectiondoes not Segment Routing WorksA routerin a SegmentRoutingnetworkis capableof selectingany path to forwardtraffic, whetherit is explicitor InteriorGatewayProtocol(IGP)shortestpath .

3 Segmentsrepresentsubpathsthat a routercan combinetoform a completerouteto a segmenthas an identifier(SegmentIdentifier)that isdistributedthroughoutthe networkusingnew IGP extensionsare equallyapplicableto IPv4and IPv6 case for traditionalMPLS networks,routersin a SegmentRouternetworkdo not requireLabelDistributionProtocol(LDP)and ResourceReservationProtocol- Traffic Engineering(RSVP-TE)to allocateor signaltheir segmentidentifiersand programtheir router(node)and each link (adjacency)has an associatedsegmentidentifier(SID).Nodeseg mentidentifiersare globallyuniqueand representthe shortestpath to a routeras determinedby the IGP. The networkadministratorallocatesa node ID to each routerfrom a the otherhand,an adjacencysegmentID is locallysignificantand representsa specificadjacency, such as egressinterface,to a neighboringrouter. Routersautomaticallygenerateadjacencyide ntifiersoutsideof the reservedblockof node IDs. In anMPLS network,a segmentidentifieris encodedas an MPLS label stackentry.

4 SegmentIDs directthe dataalonga specifiedpath. Thereare two kindsof segmentIDS: Prefix SID A segmentID that containsan IP addressprefixcalculatedby an IGP in the serviceprovidercore are node SID is a specialform of prefixSID thatcontainsthe loopbackaddressof the node as the is advertisedas an indexinto the node specificSR GlobalBlockor SRGB. AdjacencySID A segmentID that containsan advertisingrouter s adjacencyto a neighbor. AnadjacencySID is a link betweentwo adjacencySID is relativeto a specificrouter, itis locallyunique. Segment Routing Configuration Guide, Cisco IOS XE Release 3S2 Introduction to Segment RoutingHow Segment Routing WorksExamples for Segment RoutingThe followingfigureillustratesan MPLS networkwith five routersusingSegmentRouting,IS-IS,a labelrangeof 100 to 199 for node IDs, and 200 and higherfor adjacencyIDs. IS-ISwoulddistributeIP prefixreachabilityalongsidesegmentID (the MPLS label)acrossthe 1: An MPLS Network with Five Routers Using Segment RoutingIn the previousexample,any routersendingtraffic to routerE wouldpush label 103 (routerE node segmentidentifier)to forwardtraffic usingthe IS-ISshortestpath.

5 The MPLS label-swappingoperationat each hoppreserveslabel 103 until the packetarrivesat E (Figure2). On the otherhand,adjacencysegmentsbehavediffere ntly. For example,if a packetarrivesat RouterD with a top-of-stackMPLS label of 203 (D-to-Eadjacencysegmentidentifier),Route rD wouldpop the label and forwardthe traffic to 2: MPLS Label-Swapping OperationSegmentidentifierscan be combinedas an orderedlist to performtraffic segmentlist cancontainseveraladjacencysegments,sever alnode segments,or a combinationof both dependingon the previousexample,RouterA couldalternativelypush label stack(104,203)to reachRouterE usingthe shortestpath and all applicableECMPsto RouterD and then throughan explicitinterfaceonto the destination(Figure3). RouterA does not need to signalthe new path, and the state informationSegment Routing Configuration Guide, Cisco IOS XE Release 3S 3 Introduction to Segment RoutingExamples for Segment Routingremainsconstantin the ultimatelyenforcesa forwardingpolicythat determineswhichflowsdestinedto routerE are switchedthrougha 3: Router E Destination PathBenefits of Segment Routing ReadyforSDN SegmentRoutingis a compellingarchitectureconceivedto embraceSoftware-DefinedNetwork(SDN)and is the foundationfor ApplicationEngineeredRouting(AER).

6 It strikesa balancebetweennetwork-baseddistributedin telligence,such as automaticlink and node protection,andcontroller-basedcentralize dintelligence,such as traffic can providestrict networkperformanceguarantees,efficientus e of networkresources,and very high scalabilityfor networkuses minimalstate informationto meet be easilyintegratedwith a controller-basedSDN sampleSDNscenariowherethe controllerperformscentralizedoptimizatio n, this scenario,the controllerhas a completepictureof the networktopologyand routercanrequesta path to a destinationwith certaincharacteristics,for example,delay, bandwidth,diversity. Thecontrollercomputesan optimalpath and returnsthe correspondingsegmentlist, such as an MPLS label Segment Routing Configuration Guide, Cisco IOS XE Release 3S4 Introduction to Segment RoutingBenefits of Segment Routingstack,to the requestingrouter. At that point,the routercan injecttraffic with the segmentlist withoutany additionalsignalingin the 4: SDN Controller In addition,segmentlists allowcompletenetworkvirtualizationwithou taddingany applicationstate tothe state is encodedin the packetas a list of networkonly maintainssegmentstate,it can supporta large number- and a higherfrequency- of transaction-basedapplicationrequestswith outcreatingany burdenon the network.

7 Simplified Whenappliedto the MPLS data plane,SegmentRoutingoffers the abilityto tunnelMPLS services(VPN,VPLS,and VPWS)from an ingressprovideredge to an egressprovideredge withoutanyotherprotocolthan an IGP (ISISor OSPF). Simpleroperationwithoutseparateprotocols for label distribution(for example,no LDP or RSVP). No complexLDP or IGP synchronizationto troubleshoot. Betterutilizationof installedinfrastructure,for lowercapitalexpenditures(CapEx),withECMP -awareshortestpath forwarding(usingnode segmentIDs). SupportsFast Reroute (FRR) DeliverautomatedFRR for any topology. In case of link or nodefailuresin a network,MPLS uses the FRR mechanismfor segmentrouting,theconvergencetime is sub-50-msec. Large-scaleData Center- SegmentRoutingsimplifiesMPLS-enableddata centerdesignsusingBorderGatewayProtocol( BGP)RFC 3107 - IPv4 labeledunicastamongTop-of-the-Rack/Leaf/ Spineswitches. BGP distributesthe node segmentID, equivalentto IGP node SID. Any node withinthe topologyallocatesthe sameBGP segmentfor the sameswitch.

8 The same benefitsare providedas for IGP node SID: ECMPand automatedFRR (BGPPIC(PrefixIndependentConvergence). This is a buildingblockfor traffic engineering- SR TE data centerfabricoptimization. Scalable Segment Routing Configuration Guide, Cisco IOS XE Release 3S 5 Introduction to Segment RoutingBenefits of Segment RoutingAvoid thousandsof labelsin LDP database. Avoid thousandsof MPLST raffic EngineeringLSP'sin the network. Avoid thousandsof tunnelsto configure. Dual-planeNetworks SegmentRoutingprovidesa simplesolutionfor disjointnessenforcementwithina so-called dual-plane network,wherethe routeto an edge destinationfrom a givenplanestays withintheplaneunlessthe planeis partitioned. An additionalSID anycast segmentID allowsthe expressionof macropoliciessuch as: "Flow1 injectedin node A towardnode Z must go via plane1" and "Flow2 injectedin node A towardsnode Z must go via plane2." CentralizedTraffic Engineering Controllersand orchestrationplatformscan interactwith SegmentRoutingtraffic engineeringforcentralizedoptimization,su ch as WAN optimization.)

9 Networkchangessuch as congestioncan triggeran applicationto optimize(recompute)theplacementof segmentroutingtraffic engineeringtunnels. SegmentRoutingtunnelsare dynamicallyprogrammedonto the networkfrom an orchestratorusingsouthboundprotocolslike PCE. Agilenetworkprogrammingis possiblesinceSegmentRoutingtunnelsdo not requiresignalingand per-flowstate at midpointsand tail end routers. Egress PeeringTraffic Engineering(EPE) SegmentRoutingallowscentralizedEPE. A controllerinstructsan ingressprovideredge and contentsourceto use a specificegressprovideredge and specificexternalinterfaceto reacha destination. BGP peering segmentIDs are used to expresssource-routedinter-domainpaths. Controllerslearn BGP peeringSIDs and the externaltopologyof the egressborderrouterthroughBGP Link Status(BGP-LS)EPE routes. Controllersprogramingresspointswith a desiredpath. Plug-and-Playdeployment Segmentroutingtunnelsare interoperablewith existingMPLS controland data planesand can be implementedin an Routing Global BlockSegmentRoutingGlobalBlock(SRGB)is the rangeof labelsreservedfor localpropertyof an MPLS,architecture,SRGBis the set of local segmentrouting,each node can be configuredwith a differentSRGB valueand hencethe absoluteSID valueassociatedto an IGP PrefixSegmentcan changefrom node to node.

10 Segment Routing Configuration Guide, Cisco IOS XE Release 3S6 Introduction to Segment RoutingSegment Routing Global BlockThe SRGB defaultvalueis 16000to SRGBcan be configuredas follows:Device(config)#routerisis1 Device(config-isis)# Segment -routinggloba l-block4500055000 The SRGB label valueis calculatedas follows: If the platformsupports1000000labelsor more,the SRGB valueis from 900000to 900000+ 2^16. If the platformsupportsless than 1000000labels,the SRGB valueis the last 2^16 : The SRGB size cannotbe morethan 2^16. The SRGB upperboundcannotexceedthe platformcapability. The SRGB cannotbe configuredto be the samevalueas the SRGB cannotbeconfiguredfor 16000to Routing Global BlockThis chapterexplainsthe conceptof creatinga blockof labelsreservedfor a blockof reservedlabelsis knownas the SegmentRoutingGlobalBlock(SRGB).Adjacenc y Segment IdentifiersThe AdjacencySegmentIdentifier(adj-SID)is a local label that pointsto a specificinterfaceand a next hopout of that specificconfigurationis requiredto enabledover IS-ISfor an address-family, for any interfacethat IS-ISruns over, the address-familyautomaticallyallocatesan adj-SIDtowardseveryneighborout of that Segment IdentifiersA prefixsegmentidentifier(SID)identifiesa segmentroutingtunnelleadingto the destinationrepresentedby a maximumprefixSID valueis 2^16 - prefixSID is allocatedfrom the SegmentRoutingGlobalBlock(SRGB).