3.4 Convergence
3.4.1 Convergence issues
This process is both collaborative and independent; the routers share information with each other, but must individually recalculate their own routing tables. For individual routing tables to be accurate, all routers must have a common view of the network's topology. When all routers in a network agree on the topology, they have converged. Rapid convergence means rapid recovery from link failure or other network changes. Routing protocols and network designs are ultimately judged by how quickly they converge.

When routers are in the process of converging, the network is susceptible to routing problems. If some routers learn that a link is down and others incorrectly believe that the link is still up, their individual tables will be contradictory and may lead to dropped packets or devastating routing loops.

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, a significant amount of time may pass before all the routers in a network converge. Many factors affect convergence time, including the following:

  • The routing protocol used
  • A router's distance (that is, the number of 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 in relation to the topology change

The effects of some of these factors can be minimized through careful network design. For instance, a network can be designed 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. Large internetworks, however, can reduce the number of routers that must converge by using static default routes for stub networks.

Although proper network design can significantly reduce convergence time, a routing protocol's capability to update and calculate routes efficiently can also improve convergence.