Problem—Routing Loops
Each node maintains the distance from itself to each possible destination network
Slow convergence produces inconsistent routing
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
Symptom: Counting to Infinity
Packets for network 10.4.0.0 bounce between routers A, B, and C.
Hop count for network 10.4.0.0 counts to infinity.
Solution: Defining a Maximum
Define a limit on the number of hops to prevent infinite loops Solution: Split Horizon 
It is never useful to send information about a route back in the direction from which the original packet came
Solution: Route Poisoning
Routers set the distance of routes that have gone down to infinity
Solution: Poison Reverse
Poison reverse overrides split horizon
Solution: Holddown Timers
The router keeps an entry for the network possibly down state, allowing time for other routers to recompute for this topology change.
Solution: Triggered Updates
The router sends updates when a change in its routing table occurs
Routing
To route, a router needs to do the following:
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