CCNA Cisco Certified Network Associate Study Guide - FTP Server

IP Routing in Our Network 261
16 hops is deemed unreachable. In other words, after a loop of 15 hops, Network
5 will be considered down. This means that counting to infinity will
keep packets from going around the loop forever. Though this is a workable
solution, it won’t remove the routing loop itself. Packets will still go into the
loop, but instead of traveling on unchecked, they’ll 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 distancevector
network by enforcing the rule that information cannot be sent back in
the direction from which it was received. It 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 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 router 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, used with holddowns (discussed next), will speed up
convergence time because neighboring routers don’t have to wait 30 seconds
(an eternity in computer land) before advertising the poisoned route.
Holddowns
And then there are holddowns. These prevent regular update messages from
reinstating a route that has gone down. Holddowns also help prevent routes
from changing too rapidly by allowing time for either the downed route to
come back or the network to stabilize somewhat before changing to the next
best route. These also tell routers to restrict, for a specific time period, any
changes that might affect recently removed routes. This prevents inoperative
routers from being prematurely restored to other routers’ tables.