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It is virtually impossible for all routers
in a network to simultaneously detect a topology change. In fact,
depending on the routing protocol in use, as well as numerous other
factors, a considerable time delay may pass before all the routers in that
network reach a consensus, or agreement, on what the new topology is. This
delay is referred to as convergence time. The important thing to
remember is that convergence is not immediate. The only uncertainty is how
much time is required for convergence to occur.
Some factors that can exacerbate the time delay inherent in convergence
include the following:
- A router's distance (in hops) from the point of change
- The number of routers in the network that use dynamic routing
protocols
- Bandwidth and traffic load on communications links
- A router's load
- Traffic patterns vs. the topological change
- The routing protocol used
The effects of some of these factors can be minimized through careful
network engineering. For example, a network can be engineered to minimize the load on
any given router or communications link. Other factors, such
as the number of routers in the network, must be accepted as risks
inherent in a network's design. It may be possible, however, to engineer
the network such that fewer routers need to converge! For example, by
using static routes to interconnect stubs to the network (This
is one method used as an example here, not the only solution), you
reduce the number of routers that must converge. This directly reduces
convergence times. Given these factors, it is clear that the two keys to
minimizing convergence times are
- Selection of a routing protocol that can calculate routes
efficiently
- Designing the network properly
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