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Chap 11 Routing

Chap 11 Routing. Andres, Wen-Yuan Liao Department of Computer Science and Engineering De Lin Institute of Technology andres@dlit.edu.tw http://www.cse.dlit.edu.tw/~andres. Overview. Internetworking function Routing and routed protocols Track distance between locations

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Chap 11 Routing

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  1. Chap 11 Routing Andres, Wen-Yuan Liao Department of Computer Science and Engineering De Lin Institute of Technology andres@dlit.edu.tw http://www.cse.dlit.edu.tw/~andres

  2. Overview • Internetworking function • Routing and routed protocols • Track distance between locations • Distance-vector, link-state, and hybrid routing approaches

  3. Routing Basics • Path determination • Route packets from source to destination • Network and host addressing • Path selection and packet switching • Routed versus routing protocol • Network-layer protocol operations • Multiprotocol routing

  4. Path determination • Evaluate the available paths to a destination and to establish the preferred handling of a packet • Network part of IP • Layer 3

  5. Route packets • The consistency of Layer 3 addr. improves the use of bandwidth by preventing unnecessary broadcasts • Broadcasts invoke unnecessary process overhead and waste capacity

  6. Network and host addressing • The router uses the network address to identify the destination network of a packet • Assigning host addresses • Network administrator • Be partially or completely dynamic

  7. Path selection and packet switching • The router uses the network portion of the address to make path selections • The switching function: accept a packet on one interface and forward it through a second interface

  8. Routed versus routing protocol • Routed protocols define the field formats within a packet (IP): Carry data • Routing protocols provide mechanisms for sharing routing information: Maintain table

  9. Routing protocol • RIP (Routing Information Protocol) • IGRP (Interior Gateway Routing Protocol) • EIGRP (Enhanced Interior Gateway Routing Protocol) • OSPF (Open Shortest Path First)

  10. Multiprotocol routing • Routers are capable of supporting multiple independent routing protocols and maintaining routing tables for several routed protocols

  11. Why Routing Protocols are Necessary • Static versus dynamic routes • Default route • Dynamic routing • Various metrics • Three classes of routing protocols • Time to convergence

  12. Static versus dynamic routes • Static route • Be administered manually by a network administrator who enters it into a router's configuration • Dynamic route • The route knowledge: updated by a routing process

  13. Static route • Dynamic routing: reveal everything known about an inter-network • Security reasons • Hide parts of an internetwork • A stub network:A network is accessible by one path

  14. Default route • A routing table entry that directs packets to the next hop when that hop is not explicitly listed in the routing table

  15. Dynamic routing • Offers more flexibility • Loadsharing • Direct traffic from the same session over different paths in a network for better performance

  16. Dynamic routing operations • Maintenance of a routing table • Timely distribution of knowledge, in the form of routing updates, to other routers

  17. A routing protocol • How to send updates • What knowledge is contained in these updates • When to send this knowledge • How to locate recipients of the updates

  18. Three classes of routing protocols • Distance-vector routing • Determines the direction (vector) and distance to any link • Link-state (shortest path first) • Re-creates the exact topology • Balanced hybrid approach

  19. Time to convergence • Convergence • The knowledge: an accurate, consistent view of new topology • Converged • All routers in an internetwork are operating with the same knowledge • Fast convergence

  20. Distance-Vector Routing • Distance-vector routing basics • Exchange routing tables • Topology changes propagate • Routing loops • Counting to infinity • Defining a maximum • Split horizon • Hold-down timers

  21. Distance-vector routing basics • Pass periodic copies of a routing table from router to router • Do not allow a router to know the exact topology of an internetwork

  22. Exchange routing tables • Directly-connected network • A distance of 0

  23. Topology changes propagate • Topology change updates proceed step-by-step from router to router • Send its entire routing table to each of its adjacent neighbor

  24. Routing loops • A network's slow convergence on a new configuration causes inconsistent routing entries

  25. Routing loops • Packets never reach their destination but instead cycle repeatedly through the same group of network nodes

  26. Counting to infinity • The invalid updates will continue to loop until some other process stops the looping

  27. Sol: Defining a maximum • Routing metric (hop count) • Distance-vector protocols define infinity as a specific maximum no. • The distance-vector default maximum of 15 hops

  28. Sol: Split horizon • Split-horizon attempts: if a routing update about Network 1 arrives from Router A, Router B or Router D cannot send information about Network 1 back to Router A

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