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Routing Protocol Basics

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loop, but instead of traveling on unchecked, they'll just whirl around for
16 bounces and die.

Split Horizon

Another solution to the routing loop problem is called split horizon. This
reduces incorrect routing information and routing overhead in a distance-
vector network by enforcing the rule that information cannot be sent back in
the direction from which it was received.

In other words, the routing protocol differentiates which interface a net-

work route was learned on and then will not advertise that route back out
that same interface. This would have prevented Router A from sending the
updated information it received from Router B back to Router B.

Route Poisoning

Another way to avoid problems caused by inconsistent updates and stop net-
work loops is route poisoning. For example, when Network 5 goes down,
Router E initiates route poisoning by entering a table entry for Network 5 as
16, or unreachable (sometimes referred to as infinite).

By this poisoning of the route to Network 5, Router C is not susceptible

to incorrect updates about the route to Network 5. When Router C receives
a route poisoning from Router E, it sends an update, called a poison reverse,
back to Router E. This makes sure all routes on the segment have received
the poisoned route information.

Route poisoning and split horizon create a much more resilient and

dependable distance-vector network than we'd have otherwise and stop net-
work loops. However, this isn't all you need to know about loop prevention
in distance-vector networks; read on.

Holddowns

And then there are holddowns. A holddown prevents regular update mes-
sages from reinstating a route that is going up and down (called flapping).
Typically this will happen on a serial link that is losing connectivity and
then coming back up. If there was no way to stabilize this, the network
would never converge and the flapping interface could bring the net-
work down.

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