© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-1 Determining IP Routes Introducing Distance Vector Routing

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-2 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

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-3 Routers pass periodic copies of their routing table to neighboring routers and accumulate distance vectors. Distance Vector Routing Protocols

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-4 Routers discover the best path to destinations from each neighbor. Sources of Information and Discovering Routes

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-5 Selecting the Best Route with Metrics

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-6 Updates proceed step by step from router to router. Maintaining Routing Information

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-7 Each node maintains the distance from itself to each possible destination network. Inconsistent Routing Entries

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-8 Slow convergence produces inconsistent routing. Inconsistent Routing Entries (Cont.)

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-9 Router C concludes that the best path to network is through Router B. Inconsistent Routing Entries (Cont.)

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-10 Router A updates its table to reflect the new but erroneous hop count. Inconsistent Routing Entries (Cont.)

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-11 The hop count for network counts to infinity. Count to Infinity

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-12 A limit is set on the number of hops to prevent infinite loops. Defining a Maximum

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-13 Packets for network bounce (loop) between Routers B and C. Routing Loops

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-14 It is never useful to send information about a route back in the direction from which the original information came. Split Horizon

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-15 Routers advertise the distance of routes that have gone down to infinity. Route Poisoning

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-16 Poison reverse overrides split horizon. Poison Reverse

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-17 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

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-18 The router sends updates when a change in its routing table occurs. Triggered Updates

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-19 Eliminating Routing Loops

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-20 Eliminating Routing Loops (Cont.)

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-21 Eliminating Routing Loops (Cont.)

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-22 Eliminating Routing Loops (Cont.)

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-23 Eliminating Routing Loops (Cont.)

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-24 Eliminating Routing Loops (Cont.)

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-25 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.

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-26 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.

© 2006 Cisco Systems, Inc. All rights reserved. ICND v2.3—3-27