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1、MPLS Architecture Overview,Jay K,Adopted from Stefano Previdis presentation,Agenda,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 Packet,MPLS Concepts,MPLS:Multi Protocol Label SwitchingMPLS is a layer 2+swi
2、tchingDeveloped to integrate IP and ATMMPLS forwarding is done in the same way as in ATM switchesPacket forwarding is done based on Labels,MPLS Concepts,Unlike IP,classification/label can be based on:Destination Unicast address Traffic Engineering VPN QoSFEC:Forwarding Equivalence ClassA FEC can rep
3、resent a:Destination address prefix,VPN,Traffic Engineering tunnel,Class of Service.,Agenda,MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths ATM LSRs Loops and TTL LDP overview Summary,LSRs and Labels,LSR:Label Switch RouterEdge-LSR:LSRs that do label imposition and
4、 dispositionATM-LSR:An ATM switch with Label Switch Controller,LSRs and Labels,An IP routing protocol is used within the routing domain(e.g.:OSPF,i-ISIS)A label distribution protocol is used to distribute address/label mappings between adjacent neighbors The ingress LSR receives IP packets,performs
5、packet classification,assign a label,and forward the labelled packet into the MPLS network Core LSRs switch packets/cells based on the label value The egress LSR removes the label before forwarding the IP packet outside the MPLS network,LSRs and Labels,Uses new Ethertypes/PPP PIDs/SNAP values/etcMor
6、e than one Label is allowed-Label StackMPLS LSRs always forward packets based on the value of the label at the top of the stack,Label=20 bits Exp=Experimental,3 bits S=Bottom of stack,1bitTTL=Time to live,8 bits,LSRs and Labels,PPP Header,Layer 3 Header,Shim Header,PPP Header(Packet over SONET/SDH),
7、Ethernet Hdr,Layer 3 Header,Shim Header,Ethernet,FR Hdr,Layer 3 Header,Shim Header,Frame Relay,ATM Cell Header,HEC,DATA,CLP,PTI,VCI,GFC,VPI,HEC,DATA,CLP,PTI,VCI,GFC,VPI,Subsequent cells,Agenda,MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths ATM LSRs Loops and TTL L
8、DP overview Day in the Life of a Packet,Label Assignment and Distribution,Labels have link-local significanceEach LSR binds his own label mappings Each LSR assign labels to his FECs Labels are assigned and exchanged between adjacent neighboring LSR Applications may require non-adjacent neighbors,Lab
9、el Assignment and Distribution,Rtr-C is the downstream neighbor of Rtr-B for destination 171.68.10/24Rtr-B is the downstream neighbor of Rtr-A for destination 171.68.10/24LSRs know their downstream neighbors through the IP routing protocol Next-hop address is the downstream neighbor,171.68.10/24,Rtr
10、-B,Rtr-A,Rtr-C,171.68.40/24,Upstream and Downstream LSRs,Label Assignment and Distribution,LSRs distribute labels to the upstream neighbors,171.68.10/24,Rtr-B,Rtr-A,Rtr-C,171.68.40/24,IGP derived routes,Unsolicited Downstream Distribution,Label Assignment and Distribution,Upstream LSRs request label
11、s to downstream neighborsDownstream LSRs distribute labels upon request,171.68.10/24,Rtr-B,Rtr-A,Rtr-C,171.68.40/24,On-Demand Downstream Distribution,Label Assignment and Distribution,Liberal retention mode LSR retains labels from all neighborsImprove convergence time,when next-hop is again availabl
12、e after IP convergenceRequire more memory and label space Conservative retention mode LSR retains labels only from next-hops neighborsLSR discards all labels for FECs without next-hopFree memory and label space,Label Retention Modes,Label Assignment and Distribution,Label Distribution Modes,Independ
13、ent LSP controlLSR binds a Label to a FEC independently,whether or not the LSR has received a Label the next-hop for the FECThe LSR then advertises the Label to its neighbor Ordered LSP control LSR only binds and advertise a label for a particular FEC if:it is the egress LSR for that FEC or it has a
14、lready received a label binding from its next-hop,Label Assignment and Distribution,Several protocols for label exchange LDPMaps unicast IP destinations into labels RSVP,CR-LDPUsed in traffic engineering BGPExternal labels(VPN)PIMFor multicast states label mapping,Agenda,MPLS Concepts LSRs and label
15、s Label assignment and distribution Label Switch Paths ATM LSRs Loops and TTL LDP overview Day in the Life of a Packet,Label Switch Path(LSP),LSPs are derived from IGP routing informationLSPs may diverge from IGP shortest pathLSP tunnels(explicit routing)with TELSPs are unidirectionalReturn traffic
16、takes another LSP,LSP follows IGP shortest path,LSP diverges from IGP shortest path,IGP domain with a label distribution protocol,IGP domain with a label distribution protocol,Label Switch Path(LSP)Penultimate Hop Popping,The label at the top of the stack is removed(popped)by the upstream neighbor o
17、f the egress LSR The egress LSR requests the“popping”through the label distribution protocolEgress LSR advertises implicit-null label The egress LSR will not have to do a lookup and remove itself the labelOne lookup is saved in the egress LSR,Label Switch Path(LSP)Penultimate Hop Popping,0,1,Summary
18、 route for 171.68/16,0,1,171.68.10/24,Egress LSR summarises morespecific routes and advertises a label for the new FEC,Summary route is propagate through the IGP and label is assigned by each LSR,Use label“implicit-null”for FEC 171.68/16,Summary route for 171.68/16,Use label 4 for FEC 171.68/16,171.
19、68.44/24,Egress LSR needs to do an IP lookup for finding more specific routeEgress LSR need NOT receive a labelled packet,Agenda,MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths ATM LSRs Loops and TTL LDP overview Summary,ATM LSRs,ATM switches forward cells,not pack
20、ets Label Dist is Downstream on-demand,Ordered IGP label is carried in the VPI/VCI field Merging LSR:Ability to use the same label for different FECs if outgoing interface is the sameSave label space on ATM-LSRsCell interleave problem Non Merging LSR:ATM-LSR requests one label per FEC and per incomi
21、ng interface(upstream neighbors)Downstream LSR may request itself new label to its downstream neighbors,ATM LSRs Non-Merging Downstream on Demand,171.68,IPPacket,IPPacket,ATM-LSR requested additional label for same FEC in order to distinguish between incoming interfaces(Downstream on Demand),ATM LSR
22、s VC-Merging Downstream on Demand,171.68,IPPacket,IPPacket,ATM-LSR transmitted cells in sequence in order for the downstream LSR to re-assembling correctly the cells into packets,Agenda,MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths ATM LSRs Loops and TTL LDP over
23、view Summary,Loops and TTL,In IP networks TTL is used to prevent packets to travel indefinitely in the network MPLS may use same mechanism as IP,but not on all encapsulations TTL is present in the label header for PPP and LAN headers(shim headers)ATM cell header does not have TTL,Loops and TTL,LSRs
24、using ATM do not have TTL capability Some suggested options:-hop-count object in LDP-Path Vector object in LDP,Loops and TTL,TTL is decremented prior to enter the non-TTL capable LSPIf TTL is 0 the packet is discarded at the ingress pointTTL is examined at the LSP exit,IGP domain with a label distri
25、bution protocol,LSR-1,LSR-2,LSR-4,LSR-5,LSR-3,LSR-6,Egress,IP packetTTL=6,IP packetTTL=10,LSR-6-25Hops=4,Agenda,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 Packet,LDP Concepts,Label Distribution Protocol
26、Labels map to FECs for Unicast Destination Prefix LDP works between adjacent/non-adjacent peers LDP sessions are established between peers,LDP Messages,Discovery messages Used to discover and maintain the presence of new peers Hello packets(UDP)sent to all-routers multicast address Once neighbor is
27、discovered,the LDP session is established over TCP,LDP Messages,Session messages Establish,maintain and terminate LDP sessions Advertisement messages Create,modify,delete label mappings Notification messages Error signalling,MPLS Concepts LSRs and labels Label assignment and distribution Label Switc
28、h Paths ATM LSRs Loops and TTL LDP overview Day in the Life of a Packet,Agenda,Day in the life of a Packet,0,Summary route for 171.68/16,0,1,171.68.10/24,Egress LSR summarises morespecific routes and advertises a label for the new FEC,Summary route is propagate through the IGP and label is assigned
29、by each LSR,Use label“implicit-null”for FEC 171.68/16,Summary route for 171.68/16,Use label 4 for FEC 171.68/16,Egress LSR needs to do an IP lookup for finding more specific route,171.68.44/24,Use label 7 for FEC 171.68/16,CE,PE,P,P,PE,0,0,2,1,Day in the life of a Packet Basic Layout,Day in the life
30、 of a Packet Database Layout,Day in the life of a Packet,Day in the life of a Packet,Summary,LSRs forward packet based on label information IP header and forwarding decision have been de-coupled for better flexibility Label information can derive from different sourcesIP routing protocols(destinatio
31、n based unicast routing)MulticastTraffic EngineeringQoSVPN,Summary,MPLS allows flexible packet classification and network resources optimisation Labels are distributed by different protocolsLDP,RSVP,BGP,PIM Different distribution protocols may co-exist in the same LSR Label have local(LSR)significanceNo need for global(domain)wide label allocation/numbering,
