Convergence is the state of a set of routers that have the same

topological In mathematics, topology (from the Greek words , and ) is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing h ...

information about the

internetwork Internetworking is the practice of interconnecting multiple computer networks, such that any pair of hosts in the connected networks can exchange messages irrespective of their hardware-level networking technology. The resulting system of interc ...

in which they operate. For a set of routers to have ''converged'', they must have collected all available topology information from each other via the implemented

routing protocol A routing protocol specifies how routers communicate with each other to distribute information that enables them to select routes between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packets ...

, the information they gathered must not contradict any other router's topology information in the set, and it must reflect the real state of the network. In other words: in a converged network all routers "agree" on what the network topology looks like. Convergence is an important notion for a set of routers that engage in

dynamic routing Dynamic routing, also called adaptive routing, is a process where a router can forward data via a different route for a given destination based on the current conditions of the communication circuits within a system. The term is most commonly asso ...

. All

Interior Gateway Protocol An interior gateway protocol (IGP) or Interior routing protocol is a type of routing protocol used for exchanging routing table information between gateways (commonly routers) ''within'' an autonomous system (for example, a system of corpora ...

s rely on convergence to function properly. To have converged it is the normal state of an operational autonomous system. The Exterior Gateway Routing Protocol

BGP Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. BGP is classified as a path-vector routing protocol, and it mak ...

typically never converges because the Internet is too big for changes to be communicated fast enough.

Convergence process

When a routing protocol process is enabled, every participating router will attempt to exchange information about the topology of the network. The extent of this information exchange, the way it is sent and received, and the type of information required vary widely depending on the routing protocol in use, see e.g.

RIP Rest in peace (RIP), a phrase from the Latin (), is sometimes used in traditional Christian services and prayers, such as in the Catholic, Lutheran, Anglican, and Methodist denominations, to wish the soul of a decedent eternal rest and peace. ...

OSPF Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous syst ...

, BGP4. A state of convergence is achieved once all routing protocol-specific information has been distributed to all routers participating in the routing protocol process. Any change in the network that affects

routing table In computer networking, a routing table, or routing information base (RIB), is a data table stored in a router or a network host that lists the routes to particular network destinations, and in some cases, metrics (distances) associated with tho ...

s will break the convergence temporarily until this change has been successfully communicated to all other routers.

Convergence time

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

s, which should implement a mechanism that allows all routers running the protocol to quickly and reliably converge. Of course, the size of the network also plays an important role. A larger network will converge more slowly than a smaller 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 Holddown works by having each router start a timer when they first receive information about a network that is unreachable. Until the timer expires, the router will discard any subsequent route messages that indicate the route is in fact reachable ...

timers in use. OSPF is an example of a fast-converging routing protocol. A network of a few OSPF routers can converge in a matter of seconds. Certain configuration and hardware conditions will prevent a network from ever converging. For instance, a "

flapping Flapping or tapping, also known as alveolar flapping, intervocalic flapping, or ''t''-voicing, is a phonological process found in many varieties of English, especially North American, Cardiff, Ulster, Australian and New Zealand English, whereby ...

" interface (an interface that frequently changes its state between "up" and "down") might cause conflicting information to propagate throughout the network so the routers never agree on its current state. Under certain circumstances it might be desirable to withhold detailed routing information from parts of the network via

route aggregation A supernetwork, or supernet, is an Internet Protocol (IP) network that is formed by aggregation of multiple networks (or subnets) into a larger network. The new routing prefix for the aggregate network represents the constituent networks in a ...

, thereby speeding up convergence of the topological information shared by all routers.

References

{{Authority control Computer networking Routing

Convergence process

Convergence time

See also

References