Distance Vector Routing

© 2002, Cisco Systems, Inc. All rights reserved.
© 2002, Cisco Systems, Inc. All rights reserved.
2
Distance Vector Routing
Objectives
Upon completing this lesson, you will be able to:
Describe the features offered by distance vector routing protocols and give examples of each
Describe the issues associated with distance vector routing and identify solutions to those issues
Routers pass periodic copies of routing table to neighbor
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.
Inconsistent Routing Entries (Cont.)
Router A updates its table to reflect the new but erroneous hop count.
Inconsistent Routing Entries (Cont.)
Hop count for network 10.4.0.0 counts to infinity.
Count to Infinity
Define a limit 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 network’s possible down state, 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
Distance Vector Operation
Distance Vector Operation (Cont.)
Distance Vector Operation (Cont.)
Distance Vector Operation (Cont.)
Distance Vector Operation (Cont.)
Distance Vector Operation (Cont.)
Summary
Distance vector-based routing algorithms
(also known as Bellman-Ford algorithms) pass periodic copies of a routing table from router to router.
When the topology in a distance vector protocol internetwork changes, routing table updates must occur. As with the network discovery process, topology change updates proceed step-by-step from router to router.
When maintaining the routing information, inconsistencies can occur if the internetwork’s slow convergence on a new configuration causes incorrect routing entries.
Summary (Cont.)
The condition called count to infinity arises when routing table updates continue to increase the metric to a destination that cannot be reached, rather than marking the destination as unreachable.
A routing loop occurs when two or more routers have incorrect routing information indicating that a valid path to an unreachable destination exists through the other routers.
A number of techniques are available to eliminate routing loops including: split horizon, route poisoning, poison reverse, holddown timers, and triggered updates.