Dynamic Routing


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Dynamic Routing

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Many vector-distance Protocols have been superseded by link-state Protocols (such as RIP and Hello) 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