3.3 Route Calculation
3.3.2 Initiating updates
Some protocols use the passage of time to initiate routing updates. Others are event driven. That is, they are initiated whenever a topological change is detected. Holding all other variables constant, event-driven updates result in shorter convergence times than timed updates.

Timed Updates  

A timed update is a very simple mechanism. Time is decremented in a counter as it elapses. When a specified period of time has elapsed, an update is performed regardless of whether a topological change has occurred. An example of this is the Routing Information Protocol (RIP). This has two implications:

  • Many updates will be performed unnecessarily. This wastes bandwidth and router resources.
  • Convergence times can be needlessly inflated if route calculations are driven by the passing of time.

Event-Driven Updates 

Event-driven updates are a much more sophisticated means of initiating routing updates. Ostensibly, an update is initiated only when a change in the network's topology has been detected. Given that a topology change is what creates the need for convergence, this approach is obviously the more efficient one. An example of this is the OSPF routing protocol.

You can select an update initiator just by selecting a routing protocol; each protocol implements either one or the other. Therefore, this is one factor that must be considered when selecting a routing protocol