Distance vector routing protocols work by having routers periodically share their routing tables with neighboring routers. This allows each router to build up its own table indicating the distance to various destinations via each neighbor. Issues can arise from slow convergence of routing information leading to inconsistent entries. Techniques like split horizon, route poisoning, and holddown timers aim to prevent routing loops from occurring during this convergence process.

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

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

5. Selecting the
Best Route with Metrics

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

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

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

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

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

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

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

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

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

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

16. • Poison reverse overrides split horizon.
Poison Reverse

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

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

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.