1.4 Routing Protocol
1.4.4 One issue: time to convergence
The routing algorithm is fundamental to dynamic routing. Whenever the topology of the network changes because of growth, reconfiguration, or failure, the network knowledgebase must also change.

The knowledgebase needs to reflect an accurate and consistent view of the new topology. Convergence occurs when all routers in an internet are operating with the same knowledge (i.e. all routers have the same information on all the paths in the network). 

Fast convergence is a desirable network feature because it reduces the period of time that routers have outdated knowledge causing routing decisions that could be incorrect, wasteful, or both.