1 / 27

Determining IP Routes

Determining IP Routes. Introducing Distance Vector Routing. Outline. Overview Distance Vector Route Selection Routing Information Maintenance Routing Inconsistencies with Distance Vector Routing Protocols Count to Infinity Prevention Techniques to Eliminate Routing Loops

jasonljones
Download Presentation

Determining IP Routes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Determining IP Routes Introducing Distance Vector Routing

  2. Outline • Overview • Distance Vector Route Selection • Routing Information Maintenance • Routing Inconsistencies with Distance Vector Routing Protocols • Count to Infinity Prevention • Techniques to Eliminate Routing Loops • Implementation of Techniques to Eliminate Routing Loops • Summary

  3. Distance Vector Routing Protocols • Routers pass periodic copies of their routing table to neighboring routers and accumulate distance vectors.

  4. Sources of Information and Discovering Routes • Routers discover the best path to destinations from each neighbor.

  5. Selecting the Best Route with Metrics

  6. Maintaining Routing Information • Updates proceed step by step from router to router.

  7. Inconsistent Routing Entries • Each node maintains the distance from itself to each possible destination network.

  8. Inconsistent Routing Entries (Cont.) • Slow convergence produces inconsistent routing.

  9. Inconsistent Routing Entries (Cont.) • Router C concludes that the best path to network 10.4.0.0 is through Router B.

  10. Inconsistent Routing Entries (Cont.) • Router A updates its table to reflect the new but erroneous hop count.

  11. Count to Infinity • The hop count for network 10.4.0.0 counts to infinity.

  12. Defining a Maximum • A limit is set on the number of hops to prevent infinite loops.

  13. Routing Loops • Packets for network 10.4.0.0 bounce (loop) between Routers B and C.

  14. Split Horizon • It is never useful to send information about a route back in the direction from which the original information came.

  15. Route Poisoning • Routers advertise the distance of routes that have gone down to infinity.

  16. Poison Reverse • Poison reverse overrides split horizon.

  17. Holddown Timers • The router keeps an entry for the “possibly downstate” in the network, allowing time for other routers to recompute for this topology change.

  18. Triggered Updates • The router sends updates when a change in its routing table occurs.

  19. Eliminating Routing Loops

  20. Eliminating Routing Loops (Cont.)

  21. Eliminating Routing Loops (Cont.)

  22. Eliminating Routing Loops (Cont.)

  23. Eliminating Routing Loops (Cont.)

  24. Eliminating Routing Loops (Cont.)

  25. Summary • Distance vector routing protocols generate periodic routing updates addressed to directly connected routing devices. Routers running a distance vector routing protocol send periodic updates even if there are no changes in the network. • When a router receives an update from a neighbor’s router, the router compares the update with its own routing table. The router adds the cost of reaching the neighbor’s router to the path cost reported by the neighbor to establish a new metric. • Routing inconsistencies occur if slow internetwork convergence or a new configuration causes incorrect routing entries.

  26. Summary (Cont.) • Distance vector protocols define infinity as some maximum number. The routing protocol then permits the routing table update loop until the metric exceeds its maximum allowed value. • There are five techniques for eliminating routing loops on distance vector routing networks: split horizon, route poisoning, poison reverse, holddown timers, and triggered updates. • All five techniques can be used together to eliminate routing loops in area networks.

More Related