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Discussion of Dynamic Routing

Discussion of Dynamic Routing

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  • 1. Dynamic Routing Protocols A Brief Overview March 22, 2001 Professor Tom Mavroidis
  • 2. Why Dynamic Routing
    • For small networks static routing is fine
    • As the network grows it becomes impossible to keep the routing tables consistent
    • Dynamic Routing lets the network keep itself up to date
  • 3. Routing Server or Routing Daemon
    • On the internet gateway systems maintain up to date information about routes.
    • Each router propagates its routing information to other routers in the network
    • They also update their routing tables from information sent by other routers
    • This process is known as the routing server or routing daemon
  • 4. Dynamic Routing...
    • Determines the information placed placed in the routing table
    • If multiple routes are found the best route will be inserted into the routing table (choice is made according to a routing policy in the router)
  • 5. Routing Protocols
    • Two Classifications
      • link-state protocols
      • vector-distance protocols
  • 6. ICMP Redirect Messages
    • Simplest form of dynamic routing
    • Host systems are initialized with minimal routing information
    • If the router discovers better route to a particular host it send an ICMP redirect message to the router closer to the source who updates its routing table.
  • 7. Vector-Distance Protocols
    • Many vector-distance Protocols have been superseded by link-state Protocols (such as RIP and Hello)
    • Vector-distance Protocols forward routes across the network form router to router
    • Routes are sent as a table (Destination/Distance)
    • If the destination does not exist in the routing table a new route is added
  • 8. Vector-Distance continued
    • If a route already exists it is updated
      • both longer and shorter routes are updated since routes may have changed by a router coming off line or going on line
    • Rip is the most popular vector-distance protocol
      • was made available through the 4.2 BSD version of Unix
      • Limits hops to 16 (therefore limits size of network)
  • 9. Link-State Protocols
    • OSPF - Open Shortest Path First
      • Uses a graph searching algorithm
      • Updates don’t contain routes only status of directly connected links
      • When status changes the router rebuilds its graph and re-computes the routes
  • 10. Advantages to Link-state Routing
    • Each router re-computes routes independently
    • Routers are guaranteed to converge - (agree between each other)
    • Link Status messages pass across the network unchanged (easier to debug)
    • Scaleable - message size doesn’t depend on the number of networks
    • Generates less traffic on the network
  • 11. Other Interior Gateway Protocols
    • ICMP Router Discovery Protocol
      • host selects the most appropriate default route
      • work on an advertise/solicit mechanism
      • host broadcasts a router solicitation message on startup
      • Receiving routers send back an advertisement packet containing its IP address and a preference value
      • host will select and update is routing table based on the best preference value
  • 12. Other Interior Gateway Protocols
    • Hello
      • uses a time delay measurement to choose the best route
    • IDRP - InterDomain Routing Protocol
    • IS-IS - Intermediate system to Intermediate system
  • 13. Exterior Gateways
    • A collection of networks and routers under a single administrative control is known as an Autonomous System
    • Central core network provides a method of connecting constituent networks
    • Admin of the subsections is delegated to the core network
    • Packets destined out of the interior network are forwarded to the exterior gateway
  • 14. Exterior Gateways
    • Greatly increase the scalability of the Internet backbone
    • Alleviate bandwidth problems
    • Keeps the size of of the routing tables to a minimum
  • 15. Linux
    • The routed daemon implemented dynamic routing based on RIP
    • A modern replacement for routed is gated
      • supports more protocols