MPLS体系结构

1 2001, Cisco Systems. MPLS Architecture Overview Jay Kumarasamy Adopted from Stefano Previdis presentation 2 2001 Cisco Systems, Inc. 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 3 2001 Cisco Systems, Inc. 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. MPLS Concepts Unlike IP, classification/label can be based on: Destination Unicast address Traffic Engineering VPN QoS FEC: Forwarding Equivalence Class A FEC can represent a: Destination address prefix, VPN, Traffic Engineering tunnel, Class of Service. 5 2001 Cisco Systems, Inc. Agenda MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths ATM LSRs Loops and TTL LDP overview Summary 6 2001 Cisco Systems, Inc. LSRs and Labels LSR: Label Switch Router Edge-LSR: LSRs that do label imposition and disposition ATM-LSR: An ATM switch with Label Switch Controller 7 2001 Cisco Systems, Inc. LSRs and Labels IGP domain with a label distribution 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/label mappings between adjacent neighbors The ingress LSR receives IP packets, performs 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 8 2001 Cisco Systems, Inc. LSRs and Labels Uses new Ethertypes/PPP PIDs/SNAP values/etc More than one Label is allowed - Label Stack MPLS 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, 1bit TTL = Time to live, 8 bits 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 Label | Exp|S| TTL 9 2001 Cisco Systems, Inc. LSRs and Labels PPP Header Layer 3 Header Shim Header PPP Header(Packet over SONET/SDH) 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 Label HEC DATA CLP PTI VCI GFC VPI Label Subsequent cells 10 2001 Cisco Systems, Inc. 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 11 2001 Cisco Systems, Inc. Label Assignment and Distribution Labels have link-local significance Each 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 12 2001 Cisco Systems, Inc. Label Assignment and Distribution Rtr-C is the downstream neighbor of Rtr-B for destination 171.68.10/24 Rtr-B is the downstream neighbor of Rtr-A for destination 171.68.10/24 LSRs know their downstream neighbors through the IP routing protocol Next-hop address is the downstream neighbor 171.68.10/24 Rtr-B Rtr-A Rtr-C 171.68.40/24 Upstream and Downstream LSRs 13 2001 Cisco Systems, Inc. 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 Next-Hop In Lab - . Address Prefix 171.68.10 . Out I/F 1 . Out Lab 30 . In I/F 0 . Next-Hop In Lab 30 . Address Prefix 171.68.10 . Out I/F 1 . Out Lab 40 . In I/F 0 . Next-Hop In Lab 40 . Address Prefix 171.68.10 . Out I/F 1 . Out Lab - . In I/F 0 . Use label 40 for destination 171.68.10/24 Use label 30 for destination 171.68.10/24 IGP derived routes Unsolicited Downstream Distribution 14 2001 Cisco Systems, Inc. Label Assignment and Distribution Upstream LSRs request labels to downstream neighbors Downstream LSRs distribute labels upon request 171.68.10/24 Rtr-B Rtr-A Rtr-C 171.68.40/24 Use label 30 for destination 171.68.10/24 Use label 40 for destination 171.68.10/24 Request label for destination 171.68.10/24 Request label for destination 171.68.10/24 On-Demand Downstream Distribution 15 2001 Cisco Systems, Inc. Label Assignment and Distribution Liberal retention mode LSR retains labels from all neighbors Improve convergence time, when next-hop is again available after IP convergence Require more memory and label space Conservative retention mode LSR retains labels only from next-hops neighbors LSR discards all labels for FECs without next-hop Free memory and label space Label Retention Modes 16 2001 Cisco Systems, Inc. Label Assignment and Distribution Label Distribution Modes Independent LSP control LSR binds a Label to a FEC independently, whether or not the LSR has received a Label the next-hop for the FEC The 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 already received a label binding from its next-hop 17 2001 Cisco Systems, Inc. Label Assignment and Distribution Several protocols for label exchange LDP Maps unicast IP destinations into labels RSVP, CR-LDP Used in traffic engineering BGP External labels (VPN) PIM For multicast states label mapping 18 2001 Cisco Systems, Inc. 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 19 2001 Cisco Systems, Inc. Label Switch Path (LSP) LSPs are derived from IGP routing information LSPs may diverge from IGP shortest path LSP tunnels (explicit routing) with TE LSPs are unidirectional Return traffic 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 20 2001 Cisco Systems, Inc. Label Switch Path (LSP) Penultimate Hop Popping The label at the top of the stack is removed (popped) by the upstream neighbor of the egress 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 lookup and remove itself the label One lookup is saved in the egress LSR 21 2001 Cisco Systems, Inc. Label Switch Path (LSP) Penultimate Hop Popping 0 1 Summary route for 171.68/16 0 1 171.68.10/24 Next-Hop In Lab Address Prefix Out I/F Out Lab In I/F 4 171.68/16 2 pop 0 . . . . . Next-Hop In Lab Address Prefix Out I/F Out Lab In I/F - 171.68/16 1 4 0 . . . . . Egress LSR summarises more specific 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 171.68/16 171.68.44/24 Address Prefix and mask 171.68.10/24 Next-Hop 171.68.9.1 Interface Serial1 171.68.44/24 171.68.12.1 Serial2 171.68/16 . Null Egress LSR needs to do an IP lookup for finding more specific route Egress LSR need NOT receive a labelled packet 22 2001 Cisco Systems, Inc. Agenda MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths ATM LSRs Loops and TTL LDP overview Summary 23 2001 Cisco Systems, Inc. ATM LSRs ATM switches forward cells, not packets 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 same Save label space on ATM-LSRs Cell interleave problem Non Merging LSR: ATM-LSR requests one label per FEC and per incoming interface (upstream neighbors) Downstream LSR may request itself new label to its downstream neighbors 24 2001 Cisco Systems, Inc. ATM LSRs Non-Merging Downstream on Demand In Lab 5 8 . Address Prefix 171.68 171.68 . Out I/F 0 0 . Out Lab 3 4 . In I/F 1 2 . 171.68 IP Packet IP Packet ATM cell 5 ATM cell 8 ATM cell 8 ATM cell 8 ATM cell 5 ATM cell 3 ATM cell 4 ATM cell 4 ATM cell 4 ATM cell 3 ATM-LSR requested additional label for same FEC in order to distinguish between incoming interfaces (Downstream on Demand) 25 2001 Cisco Systems, Inc. ATM LSRs VC-Merging Downstream on Demand In Lab 5 8 . Address Prefix 171.68 171.68 . Out I/F 0 0 . Out Lab 3 3 . In I/F 1 2 . 171.68 IP Packet IP Packet ATM cell 5 ATM cell 8 ATM cell 8 ATM cell 8 ATM cell 5 ATM cell 3 ATM cell 3 ATM cell 3 ATM cell 3 ATM cell 3 ATM-LSR transmitted cells in sequence in order for the downstream LSR to re-assembling correctly the cells into packets 26 2001 Cisco Systems, Inc. Agenda MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths ATM LSRs Loops and TTL LDP overview Summary 27 2001 Cisco Systems, Inc. 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 28 2001 Cisco Systems, Inc. Loops and TTL LSRs using ATM do not have TTL capability Some suggested options: - hop-count object in LDP - Path Vector object in LDP 29 2001 Cisco Systems, Inc. Loops and TTL TTL is decremented prior to enter the non-TTL capable LSP If TTL is 0 the packet is discarded at the ingress point TTL is examined at the LSP exit IGP domain with a label distribution protocol LSR-1 LSR-2 LSR-4 LSR-5 LSR-3 LSR-6 Egress IP packet TTL = 6 Label = 25 IP packet TTL = 6 IP packet TTL = 10 LSR-6 - 25 Hops=4 IP packet TTL = 6 Label = 39 IP packet TTL = 6 Label = 21 30 2001 Cisco Systems, Inc. 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 31 2001 Cisco Systems, Inc. LDP Concepts Label Distribution Protocol Labels map to FECs for Unicast Destination Prefix LDP works between adjacent/non-adjacent peers LDP sessions are established between peers 32 2001 Cisco Systems, Inc. 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 discovered, the LDP session is established over TCP 33 2001 Cisco Systems, Inc. LDP Messages Session messages Establish, maintain and terminate LDP sessions Advertisement messages Create, modify, delete label mappings Notification messages Error signalling 34 2001 Cisco Systems, Inc. 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 Agenda 35 2001 Cisco Systems, Inc. Day in the life of a Packet 0 Summary route for 171.68/16 0 1 171.68.10/24 Next-Hop In Lab Address Prefix Out I/F Out Lab In I/F 7 171.68/16 2 pop 0 . . . . . Next-Hop In Lab Address Prefix Out I/F Out Lab In I/F - 171.68/16 1 4 0 . . . . . Egress LSR summarises more specific 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 Egress LSR needs to do an IP lookup for finding more specific route 171.68.44/24 Address Prefix and mask 171.68.10/24 Next-Hop 171.68.9.1 Interface Serial1 171.68.44/24 171.68.12.1 Serial2 171.68/16 . Null Use label 7 for FEC 171.68/16 Next-Hop In Lab Address Prefix Out I/F Out Lab In I/F 4 171.68/16 1 7 0 . . . . . CE PE P P PE 0 0 2 1 36 2001 Cisco Systems, Inc. Day in the life of a Packet Basic Layout IP Routing ProtocolsLabel Distribution ProtocolIP Routing TableForward Information Block (FIB)Label Forward Information Block(LFIB)Control PlaneForwarding PlaneIncoming IP PacketsOutgoing IP PacketsOutgoing Labelled PacketsIncoming LabelledPacketsRouting ExchangeLabel Binding ExchangeLabel RemovedL3 lookup37 2001 Cisco Systems, Inc. Day in the life of a Packet Database Layout OSPFISISBGPfast-adjacencyfasttag-rewritetag_infoFIBTFIBtfib_entrytag_rewriteRouting TableLDPTIBtfib_entrytfib_entryloadinfotag_infooutput-ifencapsincoming-tagoutgoing-tagtfib_entrytag_rewrite Incoming tagroute-tag-changefind-route-tagDest. IP addressincoming-tagtag_rewrite tag_hashreq_all_tagsIDB vectorsip_turbo_fstag_optimum_fsip2_tag_optimum_fs38 2001 Cisco Systems, Inc. Day in the life of a Packet IPMPLSIn-boundinterface driverchecksencaps,invokeshandler thruIDB vectorscheckearlyfeatures- bgp-based policy mapping (QPPB) and set qos-group ID- authentication proxy- check for input ACLs- check for crypto- check for setting input QoS marking- Input police/rate-input- check for NAT (outside - inside- check policy routing- check for WEB cache redirectionFIBlookupadja-cencylookup- check for NAT (inside - outside)- perform QoS classification- check for crypto- output ACL check- check for setting output QoS marking- QoS - WFQ- output police/rate-limitingIPMPLSpostlookupfeatureslabelimpositionsend IPpacketIPfragment?- compute length of tag(s)to be copied & check iffrag. is required.- copy tos field- set ttl- copy the label(s)- if (features) - perform output QoS classification - check for setting output QoS marking - do output rate-limiting - check for multi-vc - fragment, if necessary- send labelled packetIPMPLSprocesslabelledpacket39 2001 Cisco Systems, Inc. Day in the life of a Packet In-bound interfacedriver checksencaps, invokeshandler thru IDBvectorscheckearlyfeaturesIPMPLSprocesslabelledpacketcheckearlyfeatures- Input QoS Classification- Setting input QoS marking- Do input rate-limit/policingtagswitchinline- get label header- from tfib get the tag entry- get rewrite info from the tag entrytagforwardinline- update TTL value- update EXP value- SWAP/POP label, process inner label.- handle multi-vc CoScheckpostfeaturesnorewriteinfo, doLayer-3lookup- Output QoS Classification- Setting output QoS marking- Do output rate-limit/policingtransmitpacket- Platform specific WRED done in the final transmit path40 2001 Cisco Systems, Inc. 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 sources IP routing protocols (destination based unicast routing) Multicast Traffic Engineering QoS VPN 41 2001 Cisco Systems, Inc. Summary MPLS allows flexible packet classification and network resources optimisation Labels are distributed by different protocols LDP, RSVP, BGP, PIM Different distribution protocols may co-exist in the same LSR Label have local (LSR) significance No need for global (domain) wide label allocation/numbering 42 2001 Cisco Systems, Inc. Thank You!Thank You!