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| 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 topology. When all routers in a network agree on the topology they are considered to have converged. Rapid convergence means rapid recovery from link failure or other network changes. Routing protocols and network designs are ultimately judged by how rapidly they converge.

When routers are in the process of convergence, the network is susceptible to routing problems. Some routers learn that a link is down while others incorrectly believe that the link is still up. If this happens the 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. Dependent on the routing protocol in use, a significant amount of time may pass before all the routers in a network converge. Factors affecting the convergence time include the following:

- The routing protocol used
- The distance of the router, or the number of hops from the point of change
- The number of routers in the network that use dynamic routing protocols
- The bandwidth and traffic load on communications links
- The load on the router
- The traffic patterns in relation to the topology change

The effects of some of these factors can be minimized through careful network design. A network could 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 to the design of a network. Large internetworks 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, the capability of the routing protocol to update and calculate routes efficiently may also improve convergence.