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Understanding Routing

Understanding Routing. Agenda. What Is Routing? Network Addressing Routing Protocols. What is Routing?. What Is Routing?. Routing is: Finding a path between a source and destination (path determination)

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Understanding Routing

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  1. Understanding Routing

  2. Agenda • What Is Routing? • Network Addressing • Routing Protocols

  3. What is Routing?

  4. What Is Routing? • Routing is: • Finding a path between a source and destination (path determination) • Moving information across an internetwork from a source to a destination (switching*) • Very complex in large networks because of the many potential intermediate nodes • A router is: • A network layer device that forwards packets from one network to another and determines the optimal path for forwarding network traffic * The term “switching”, when used to describe a router’s function, is different from a switch (the network device).

  5. Routing Table 192.168.3.0 Frame Relay 192.168.1.0 Ethernet 192.168.2.0 FDDI Routers—Layer 3 Remote Location Network 192.168.3.0 Frame Relay Main Site Network 192.168.2.0 FDDI Network 192.168.1.0 Ethernet

  6. Where are Routers Used? • LAN-to-LAN connectivity • LAN-to-WAN connectivity • Remote access

  7. X Y C C A A B Application B Presentation Presentation B A C Session Application Transport Network Network Network Network Session Data Link Data Link Data Link Data Link Transport Physical Physical Physical Physical Network Data Link Physical LAN-to-LAN Connectivity • Routers encapsulate and de-encapsulate data packets as they are transferred from system X to system Y

  8. Routers find the best path through the network Routing tables contain route information Network addresses represent the path of media connections to a destination Path Determination 5 2 9 8 Which Path? 4 10 11 6 1 3 7

  9. Routing Algorithm Goals • Optimality • Selecting the best route based on metrics and metric weightings used in the calculation • Simplicity and low overhead • Efficient routing algorithm functionality with a minimum of software and utilization overhead • Robustness and stability • Correct performance in the face of unusual or unforeseen circumstances (e.g., high load) • Rapid convergence • Fast agreement, by all routers, on optimal routes • Flexibility • Quick and accurate adaptation to changes in router availability, bandwidth, queue size, etc.

  10. Routing Metrics • Path length • Total hop count or sum of cost per network link • Reliability • Dependability (bit error rate) of each network link • Delay • Useful because it depends on bandwidth, queues, network congestion, and physical distance • Communication cost • Operating expenses of links (private versus public) • Bandwidth and load

  11. Network Addressing

  12. 2 Network and Node Addresses Network Node 1 1 2.1 2 1.2 1.1 3 3.1 1 2 1 1.3 3 1 3 • Network address—Path part used by the router • Node address—Specific port or device on the network

  13. Addressing Examples Network Node/Host Protocol Address Address General 1. 4 TCP/IP 10. 8.2.48 Novell IPX 1aceb0b 0000.0c00.6e25 AppleTalk 10. 1. X.25 DNIC NTN NTN: National Terminal Number

  14. Subdividing address space into smaller blocks Helps organize network Security (keeps HR separately addressable) Scalability—Keeps traffic to appropriate segments Allows single, summarized routing entry (131.108.0.0) to be advertised to external networks Specific route entries (131.108.8.0) required only for routers in the subnetted block 131.108. 5 .0 131.108. 9 .0 131.108. 10 .0 Subnetwork Addressing INTERNET 131.108.0.0 131.108.1.0 131.108. 2 .0 131.108. 3 .0 HR 131.108. 4 .0 131.108. 6 .0 131.108. 8 .0 Manufacturing 131.108. 7 .0 R&D

  15. Routing Algorithm Types • Single-path versus multi-path • Flat versus hierarchical • Host-intelligent versus router-intelligent • Intradomain versus interdomain • Static versus dynamic routing • Link state versus distance vector

  16. Static Routing • Manual table updates by a network administrator • Benefits • Reflects administrator’s special topology knowledge • Private—Not conveyed to other routers in updates • Avoids the overhead of dynamic routing • Stub network • When a node is accessible by only one path, a static route is sufficient • Point-to-point or circuit-switched connection A B “Stub” Network

  17. A B X D C Dynamic Routing • Most internetworks use dynamic routing A B X D C A network change blocks the established path... …and an alternate route is found dynamically.

  18. Distance Vector versus Link State • Distance vector • Sends routing table info only to neighbors, so change communication may need one min/router • Also called “routing by rumor” • Easy to configure, but slow • Link state • Floods routing information about itself to all nodes, so changes are known immediately • Efficient, but complex to configure • Cisco’s EIGRP hybrid • Efficient and easy to configure

  19. Routing Protocols

  20. Routed versus Routing Protocols • Routed protocols used between routers to direct user traffic; also called network protocols • Examples: IP, IPX, DECnet, AppleTalk, NetWare, OSI, VINES • Routing protocols used between routers to maintain routing tables • Examples: RIP, IGRP, OSPF, BGP, EIGRP Network Destination Exit Port Protocol Network to Use Protocol name 1.0 1.1 2.0 2.1 3.0 3.1

  21. Routing Protocol Evolutions • EIGRP • Hybrid protocol • Developed by Cisco • Superior convergence and operating efficiency • Merges benefits of link state & distance vector • IGRP • Distance vector • Developed by Cisco • Addresses problems in large, heterogeneous networks • RIP • Distance vector • Most common IGP • Uses hop count • OSPF • Link state, hierarchical • Successor to RIP • Uses least-cost routing, multipath routing, and load balancing • Derived from IS-IS Distance Vector Hybrid Link State

  22. RIP and IGRP 19.2 k 19.2 k 64k 64k 64k 64k 64k 64k RIP Industry standard that selects the path with the fewest hops IGRP Cisco protocol that selects the fastest path (using load, distance, etc.)

  23. OSPF and EIGRP Aspect OSPF EIGRP Topology Hierarchical Not restricted Memory & CPU requirements High Moderate Routing table size Large Moderate Controlling body Industry standard Cisco proprietary Convergence Fast Fast Configuration Difficult Easy Supported protocols IP IP IPX AppleTalk

  24. Summary • Routers move data across networks from a source to a destination • Routers determine the optimal path for forwarding network traffic • Routing protocols communicate reachability information between routers

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