© 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—3-1

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

• Implementation of Techniques to Eliminate Routing Loops

• Summary


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

Distance Vector Routing Protocols


• Routers discover the best path to destinations from each neighbor.

Sources of Information and Discovering Routes


Selecting the Best Route with Metrics


• Updates proceed step by step from router to router.

Maintaining Routing Information


• Each node maintains the distance from itself to each possible destination network.

Inconsistent Routing Entries


• Slow convergence produces inconsistent routing.

Inconsistent Routing Entries (Cont.)


• Router C concludes that the best path to network 10.4.0.0 is through Router B.



• Router A updates its table to reflect the new but erroneous hop count.



• The hop count for network 10.4.0.0 counts to infinity.

Count to Infinity


• A limit is set on the number of hops to prevent infinite loops.

Defining a Maximum


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

Routing Loops


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

Split Horizon


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

Route Poisoning


• Poison reverse overrides split horizon.

Poison Reverse


• 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


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

Triggered Updates


Eliminating Routing Loops


Eliminating Routing Loops (Cont.)










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.


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.


Documents

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