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IP Routing Principles

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IP Routing Principles

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  1. IP Routing Principles

  2. Application Application Presentation Presentation Session Session Transport Transport Network Network Network Network Network Data Link Data Link Data Link Data Link Data Link Physical Physical Physical Physical Physical Network-Layer Protocol Operations X Y C A B Y X • Each router provides network layer (routing) services B A C

  3. Static versus Dynamic Routes • Static route • Uses a protocol route that a network administrator enters into the router • Dynamic route • Uses a route that a network routing protocol adjusts automatically for topology or traffic changes

  4. Static Route Example Point-to-point or circuit-switched connection A Only a single network connection with no need for routing updates • Fixed route to address reflects administrator’s knowledge B “Stub” Network

  5. Company X Internet 172.34.56.0 A B C 10.0.0.0 Routing Table No entry for destination net Try router B default route Default (static) Route Example • Use if next hop is not explicitly listed in the routing table

  6. Dynamic routing

  7. Adapting to Topology Change B A D C

  8. Adapting to Topology Change B A X • Can an alternate route substitute for a failed route? D C

  9. Adapting to Topology Change B A X • Can an alternate route substitute for a failed route? D C

  10. Network Dynamic Routing Operations Routing Protocol A router passes routing information to its neighbors Routing Table • Routing protocol maintains and distributes routing information

  11. Network Dynamic Routing Operations Routing Protocol Routing Protocol A router passes routing information to its neighbors Routing Table Routing Table • Routing protocol maintains and distributes routing information

  12. Representing Distance with Metrics A 56 Hop count Ticks Cost T1 56 • Information used to select the best path for routing T1 B

  13. Representing Distance with Metrics A Bandwidth Delay Load Reliability MTU 56 Hop count Ticks Cost T1 56 • Information used to select the best path for routing T1 B

  14. B A C D Classes of Routing Protocols Distance Vector Hybrid Routing B Link State A C D

  15. One Issue: Time to Convergence • Convergence occurs when all routers use a consistent perspective of network topology • After a topology changes, routers must recompute routes, which disrupts routing • The process and time required for router reconvergence varies in routing protocols

  16. B A C D Distance Vector Concept A • Pass periodic copies of routing table to neighbor routers and accumulate distance vectors D C B Routing Table Routing Table Routing Table Routing Table

  17. B C A D Link-State Concept Link-State Packets Topological Database Routing Table • After initial flood, pass small event-triggered link-state updates to all other routers SPF Algorithm Shortest Path First Tree

  18. Comparing Distance Vector Routing to Link-State Routing Distance Vector Link-State Views net topology from neighbor’s perspective Adds hops from router to router Frequent, periodic updates: slow convergence Passes copies of routing table to neighbor routers Gets common view of entire network topology Calculates the shortest path to other routers Event-triggered updates: faster convergence Passes link-state routing updates to other routers

  19. Choose a routing path based on distance vectors Converge rapidly using change-based updates Balanced Hybrid Routing Hybrid Routing • Share attributes of both distance-vector and link-state routing

  20. Static Routing • Advantages: • predictable • no additional CPU overhead • no additional network overhead • easy to configure • Disadvantages: • does not scale • hard to maintain • does not adapt to network changes

  21. Dynamic Routing • Advantages: • scalability • adaptability • Disadvantages: • increased complexity • increased bandwidth overhead • increased resource usage

  22. Dynamic Routing • Moral of the story: • Use static routing where you can,use dynamic routing only where you must

  23. Autonomous Systems • Internetwork • The big “I” Internet • AS = a group of routers and their networks (administered by the same owner) • AS peering

  24. Routing Protocols • Where is it used? • Interior Gateway Protocols (IGP) • RIP • OSPF • EIGRP • (IS-IS) • Exterior Gateway Protocols (EGP) • BGP

  25. Routing Protocols • How does it work? • Distance-Vector Protocols • RIP • EIGRP • BGP • Link State • OSPF

  26. Protocol Features • RIPv2 • hop count metric • unreliable transport • passive RIP • simple

  27. Protocol Features • OSPF • full CIDR support • trust • route injection • hierarchical routing • a smarter protocol

  28. Protocol Features • EIGRP • more information in advertisements • improved convergence properties • partial and incremental updates • no support for areas

  29. Protocol Features • BGP • BGP-4: RFC 1771 • created to support CIDR • even more information • Policy routing • Reliable transport • can also be used as an IGP (IBGP) • high overhead

  30. Real World Routing • multiple routing protocols are generally necessary • interior routing is a LOT different than exterior routing • multi-protocol issues

  31. ASBR ASBR Implementation Considerations EIGRP RIP 172.16 172.16 RIP 172.16.0.0 AS 300EIGRP RIP EIGRP 172.16 172.16 • Routing feedback • Suboptimal path selection • Routing loops • Incompatible routing information • Inconsistent convergence time

  32. 172.16.12.1 172.16.3.2 Trans 172.16.2.2 172.16.2.1 T-1 172.16.3.1 How can we prevent routing update traffic from crossing some of these links? 172.16.1.1 172.16.1.2 Cen R200 172.16.4.1 172.16.5.1 172.16.7.2 172.16.4.2 T-1Frame Relay 64Kb Rem 172.16.11.1 172.16.7.1 172.16.5.2 64Kb R100 R300 172.16.6.1 172.16.6.2 172.16.9.1 172.16.10.1 Controlling Routing Update Traffic

  33. Defining Distance • Different protocols use different metrics • Metrics are difficult to compare algorithmically

  34. Defining Distance • Different protocols use different metrics • Metrics are difficult to compare algorithmically • Therefore, need a selection process: • 1—Which protocol do you believe the most? • 2—Then decide which metric is the best

  35. Route Source Default Distance Connected Interface 0 Static Route 1 Enhanced IGRP Summary Route 5 External BGP 20 Internal Enhanced IGRP 90 IGRP 100 OSPF 110 IS-IS 115 RIP 120 EGP 140 External Enhanced IGRP 170 Internal BGP 200 Unknown 255 What Protocol to Believe?

  36. Routers! • Cisco CRS-1 IP/MPLS Router • 92Tbps total switching capacity • 1152 x OC-768c/STM-256c (40Gbps) Line cards • 707 Kg, 15 KW per chassis • IPv4, IPv6, MPLS

  37. Routers! • Juniper T640 Internet Routing Node • 640 Gbps total switching capacity • 770 Million packet per second forwarding • 40 Gbps per slot (4 * OC-192c, 1 * OC-768c) • 8 slots per rack

  38. Routers! • Foundry NetIron 1500 Internet Router • 480 Gbps total switching capacity • 178 Million packet per second forwarding • 10 Gbps per slot (1 * 10G Ethernet) • 15 slots per rack