1 / 42

MPLS Architecture Overview

MPLS Architecture Overview. Jay Kumarasamy jayk@cisco.com. Adopted from Stefano Previdi’s presentation. Agenda. MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths ATM LSRs Loops and TTL LDP overview

nasia
Download Presentation

MPLS Architecture Overview

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. MPLS Architecture Overview Jay Kumarasamyjayk@cisco.com Adopted from Stefano Previdi’s presentation

  2. 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. 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. 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. Agenda • MPLS Concepts • LSRs and labels • Label assignment and distribution • Label Switch Paths • ATM LSRs • Loops and TTL • LDP overview • Summary

  6. 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. IGP domain with a label distribution protocol 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 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. 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 LSRs and Labels Label = 20 bits Exp = Experimental, 3 bits S = Bottom of stack, 1bit TTL = Time to live, 8 bits • 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

  9. LSRs and Labels Label Label PPP Header(Packet over SONET/SDH) PPP Header Shim Header Layer 3 Header Ethernet Ethernet Hdr Shim Header Layer 3 Header Frame Relay FR Hdr Shim Header Layer 3 Header GFC VPI VCI PTI CLP HEC DATA ATM Cell Header Subsequent cells GFC VPI VCI PTI CLP HEC DATA

  10. 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. 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. Label Assignment and Distribution Upstream and Downstream LSRs • 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 171.68.40/24 171.68.10/24 Rtr-A Rtr-B Rtr-C

  13. Use label 30 for destination 171.68.10/24 Use label 40 for destination 171.68.10/24 In I/F In I/F In I/F In Lab In Lab In Lab Address Prefix Address Prefix Address Prefix Out I/F Out I/F Out I/F Out Lab Out Lab Out Lab 0 0 0 - 40 30 171.68.10 171.68.10 171.68.10 1 1 1 40 - 30 Next-Hop Next-Hop Next-Hop ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... Label Assignment and Distribution Unsolicited Downstream Distribution • LSRs distribute labels to the upstream neighbors 171.68.40/24 171.68.10/24 Rtr-A Rtr-B Rtr-C IGP derived routes

  14. 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 Label Assignment and Distribution On-Demand Downstream Distribution 171.68.10/24 171.68.40/24 Rtr-A Rtr-B Rtr-C • Upstream LSRs request labels to downstream neighbors • Downstream LSRs distribute labels upon request

  15. Label Assignment and Distribution • Label Retention Modes • 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

  16. 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. 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. 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. Label Switch Path (LSP) IGP domain with a label distribution protocol IGP domain with a label distribution protocol • 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

  20. 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. Address Prefix and mask Next-Hop Interface In I/F In I/F In Lab In Lab Address Prefix Address Prefix Out I/F Out I/F Out Lab Out Lab Serial1 171.68.10/24 171.68.9.1 0 0 4 - 171.68/16 171.68/16 2 1 4 pop Next-Hop Next-Hop 171.68.44/24 171.68.12.1 Serial2 ... ... ... ... ... ... ... ... ... ... 171.68/16 ... Null Label Switch Path (LSP) Penultimate Hop Popping • Summary route for 171.68/16 • Summary route for 171.68/16 0 0 1 1 171.68.44/24 • Use label 4 for FEC 171.68/16 • Use label “implicit-null” for FEC 171.68/16 171.68.10/24 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 • Egress LSR needs to do an IP lookup for finding more specific route • Egress LSR need NOT receive a labelled packet

  22. Agenda • MPLS Concepts • LSRs and labels • Label assignment and distribution • Label Switch Paths • ATM LSRs • Loops and TTL • LDP overview • Summary

  23. 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. In I/F In Lab Address Prefix Out I/F Out Lab 1 5 171.68 0 3 2 8 171.68 0 4 ... ... ... ... ... 5 8 8 3 8 4 4 3 4 5 • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell ATM LSRs Non-Merging Downstream on Demand ATM-LSR requested additional label for same FEC in order to distinguish between incoming interfaces (Downstream on Demand) • IPPacket 171.68 • IPPacket

  25. ATM LSRs VC-Merging Downstream on Demand In I/F In Lab Address Prefix Out I/F Out Lab 1 5 171.68 0 3 2 8 171.68 0 3 ... ... ... ... ... 5 3 3 3 3 3 8 8 8 5 • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell ATM-LSR transmitted cells in sequence in order for the downstream LSR to re-assembling correctly the cells into packets • IPPacket 171.68 • IPPacket

  26. Agenda • MPLS Concepts • LSRs and labels • Label assignment and distribution • Label Switch Paths • ATM LSRs • Loops and TTL • LDP overview • Summary

  27. 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. 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. Loops and TTL Label = 25 Label = 21 Label = 39 IP packetTTL = 6 IP packetTTL = 6 IP packetTTL = 6 LSR-1 LSR-3 LSR-2 IP packetTTL = 10 LSR-6 LSR-6 --> 25Hops=4 IP packetTTL = 6 IGP domain with a label distribution protocol Egress LSR-5 LSR-4 • 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

  30. 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. 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. 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. LDP Messages • Session messages • Establish, maintain and terminate LDP sessions • Advertisement messages • Create, modify, delete label mappings • Notification messages • Error signalling

  34. 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

  35. Day in the life of a Packet Address Prefix and mask Next-Hop Interface In I/F In I/F In I/F In Lab In Lab In Lab Address Prefix Address Prefix Address Prefix Out I/F Out I/F Out I/F Out Lab Out Lab Out Lab Serial1 171.68.10/24 171.68.9.1 0 0 0 4 - 7 171.68/16 171.68/16 171.68/16 2 1 1 7 4 pop Next-Hop Next-Hop Next-Hop 171.68.44/24 171.68.12.1 Serial2 ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... 171.68/16 ... Null P P 1 1 2 PE 0 PE 0 0 • Use label “implicit-null” for FEC 171.68/16 0 • Use label 4 for FEC 171.68/16 • Use label 7 for FEC 171.68/16 171.68.44/24 • Summary route for 171.68/16 • Summary route for 171.68/16 CE 171.68.10/24 Summary route is propagate through the IGP and label is assigned by each LSR Egress LSR summarises morespecific routes and advertises a label for the new FEC • Egress LSR needs to do an IP lookup for finding more specific route

  36. Day in the life of a Packet Basic Layout

  37. Day in the life of a Packet Database Layout

  38. Day in the life of a Packet

  39. Day in the life of a Packet

  40. 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. 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

More Related