3.3 Route Calculation
3.3.1 Communication among routers
As demonstrated through the examples in the preceding section, convergence is absolutely critical to a network's capability to respond to operational fluctuations. The key factor in convergence is communications among the routers in the network. Routing protocols are responsible for providing this function. Specifically, these protocols are designed to enable routers to share information about routes to the various destinations within the network.

Note:  Route Flapping, one symptom of network instability that may arise is known as route flapping. Route flapping is just the rapid vacillation between two, or more, routes. Flapping happens during a topology change. All the routers in the network must converge on a consensus of the new topology. Toward this end, they begin sharing routing information.

In an unstable network, a router (or routers) may be unable to decide on a route to a destination. Remember that during convergence a router may alter its primary route to any given destination as a result of the last- received update. In complex, but unstable networks with redundant routes, a router may find itself deciding on a different route to a given destination every time it receives an update. Each update nullifies the previous decision and triggers another update to the other routers. These other routers, in turn, adjust their own routing tables and generate "new" updates. This vicious cycle is known as flapping. You may find it necessary to power down affected routers and slowly develop convergence in your network, one router at a time.

Unfortunately, all routing protocols are not created equal. In fact, one of the best ways to assess the suitability of a routing protocol is to evaluate its capabilities to calculate routes and converge relative to other routing protocols. It should be obvious from the previous list of factors that convergence times may be difficult for you to calculate with any degree of certainty. Your router vendor may be able to assist you with this process, even if the vendor provides you with general estimates only.

A routing protocol's convergence capability is a function of its capability to calculate routes. The efficacy of a routing protocol's route calculation - is based on several factors: 

  • Whether the protocol calculates, and stores, multiple routes to each destination
  • The manner in which routing updates are initiated
  • The metrics used to calculate distances or costs

Storing Multiple Routes

Some routing protocols attempt to improve their operational efficiency by only recording a single route (ideally, the best route) to each known destination. The drawback to this approach is that when a topology change occurs, each router must calculate a new route through the network for the impacted destinations. Examples will be seen in the labs and upcoming Routing Protocol Chapters.

Other protocols accept the processing overheads that accompany larger routing table sizes and store multiple routes to each destination. Under normal operating conditions, multiple routes enable the router to balance traffic loads across multiple links. If, or when, a topology change occurs, the routers already have alternative routes to the impacted destinations in their routing tables. Having an alternative route already mapped out does not necessarily accelerate the convergence process. It does, however, enable networks to more gracefully sustain topology changes.