3.2 Convergence
3.2.3 Convergence time

It is virtually impossible for all routers in a network to simultaneously detect a topology change. In fact, depending on the routing protocol in use, as well as numerous other factors, a considerable time delay may pass before all the routers in that network reach a consensus, or agreement, on what the new topology is. This delay is referred to as convergence time. The important thing to remember is that convergence is not immediate. The only uncertainty is how much time is required for convergence to occur.

Some factors that can exacerbate the time delay inherent in convergence include the following:

  • A router's distance (in hops) from the point of change
  • The number of routers in the network that use dynamic routing protocols
  • Bandwidth and traffic load on communications links
  • A router's load
  • Traffic patterns vs. the topological change
  • The routing protocol used

The effects of some of these factors can be minimized through careful network engineering. For example, a network can be engineered to minimize the load on any given router or communications link. Other factors, such as the number of routers in the network, must be accepted as risks inherent in a network's design. It may be possible, however, to engineer the network such that fewer routers need to converge! For example, by using static routes to interconnect stubs to the network (This is one method used as an example here, not the only solution), you reduce the number of routers that must converge. This directly reduces convergence times. Given these factors, it is clear that the two keys to minimizing convergence times are

  • Selection of a routing protocol that can calculate routes efficiently
  • Designing the network properly