Convergence is the process in which routers recognize a change in the network, communicate to all the routers about this change, make the required calculations for best routes to destinations on the network, and then update all routers to have a complete and precise view of this recalculated topology, thereby stabilizing the network.
When a network is converged, this is the normal operational state where all routers have the same view of the topology.
Convergence time is the amount of time it takes to stabilize after a change has occurred, and is affected by the selection and configuration of particular routing protocols, and their implementation.
There are various tools to aid administrators to optimize convergence times:
Route timer tuning: adjusting timers to be more aggressive in response to network link failures, such as reducing EIGRP’s hello and hold timers.
Route summarization: summarizing routes can minimize the number of route calculations required for convergence, thereby expediting the process.
Route Flap Dampening: Typically used in large or ISP environments, route dampening can be enabled to enact suppress penalties on routes that flap. Once the suppress penalty is reached, the route is removed from the table until a later time where the penalty has decayed to a reuse threshold. This limits impact upon a router’s CPU and other resources.
Other convergence tools: Bi-Directional Forwarding Detection (BFD), Stateful Switch Over (SSO), Graceful Restart (NSF), BGP Fast Re-Route (FRR), and BGP Prefix Independent Convergence (PIC).
Stuck-in-Active: Journal of an IT-Network Administrator 