Convergence time is a measure of how fast a group of routers reach the state of convergence. It is one of the main design goals and an important performance indicator for routing protocols to implement a mechanism that allows all routers running this protocol to quickly and reliably converge. Of course, the size of the network also plays an important role, a larger network will converge slower than a small one.
RIP is a routing protocol that converges so slowly that even a network of a few routers can take a couple of minutes to converge. In case of a new route being advertised, triggered updates can speed up RIP's convergence but to flush a route that previously existed takes longer due to the holddown timers in use. OSPF is an example of a fast-converging routing protocol. A network of a few routers can converge in a matter of seconds.
Certain configuration and hardware conditions will prevent a network from ever converging. For instance, a "flapping" interface (an interface that frequently changes its state between "up" and "down") might cause conflicting information to propagate the network so that routers never agree on its current state. Under certain circumstances it might be desired to withhold routing information details from parts of the network via route aggregation, thereby speeding up convergence of the topological information shared by all routers.
Read more about this topic: Convergence (routing)
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