Convergence Definition

Convergence is the process of routers agreeing on optimal routes for forwarding packets and thereby completing the updating of their routing tables.

A router is an electronic device and/or software that connects at least two networks and forwards packets among them according to the information in the packet headers and routing tables. A routing table is a database in a router that stores and updates the locations (i.e., network addresses) of other network devices and the most efficient routes to them. A packet is the fundamental unit of information transport in all modern computer networks, and increasingly in other communications networks as well.

Convergence occurs as a result of a change in network topology, i.e., a link becoming available or unavailable. When this occurs, each router independently runs a routing algorithm to recalculate metrics and build a new routing table based on this information. Once all the routing tables have been updated, convergence is complete.

Metrics are any value used by routing algorithms to determine the best route among multiple routes to a destination. They are typically based on such information as bandwidth, hop count, path cost, delay, load, MTU (maximum transmission unit), reliability and communications cost. A hop is the number of links or routers that are crossed en route to the destination. MTU is the largest packet size, measured in bytes, that can be transmitted over a network.

The time required before all of the routers can reach a consensus regarding the new topology, called convergence time, depends on (1) the number of routers in the network that use dynamic routing protocols, (2) the distance of routers (measured in hops) from the point of change, (3) the bandwidth and traffic load on communications links, (4) the load on the routers, (5) traffic patterns vis-a-vis the change and (6) the routing protocol used. Among the ways in which convergence time can be minimized are using improved convergence algorithms and designing the network so that fewer routers need to converge and so that the load on any given router or communications link is minimized.

Convergence is a dynamic routing process as opposed to static routing. In the latter, routes are programmed into routers by humans. It is suitable for small networks or when dedicated links exist between networks.

Convergence is necessary because routers are intelligent devices that are capable of making their own routing decisions. This distributed intelligence is usually a huge advantage, because it allows large networks to be vastly faster, more robust and more efficient than would be possible with direct human intervention. However, in some cases, particularly with poor network design, overloading and/or malicious attacks, it can lead to instability.

The term convergence also has a number of other meanings. In an electronics context it can mean digital convergence, which is the trends for various information and communications technologies to become digital, and it can refer to the trend for communications technologies to be based on packet switching and operate over a common network infrastructure.

Created November 18, 2005.
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