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This process is both collaborative and independent; the routers share
information with each other, but must individually recalculate their own
routing tables. For individual routing tables to be accurate, all routers must have a common view of the network's topology. When all
routers in a network agree on the topology, they have converged. Rapid
convergence means rapid recovery from link failure or other network
changes. Routing protocols and network designs are ultimately judged by
how quickly they converge.
When routers are in the process of
converging, the network is susceptible to routing problems. If some
routers learn that a link is down and others incorrectly believe that the
link is still up, their individual tables will be contradictory and may
lead to dropped packets or devastating routing loops.
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, a significant amount of time may pass before all the routers in a
network converge. Many factors affect convergence time, including the
following:
- The routing protocol used
- A router's distance (that is, the number
of 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 in relation to the
topology change
The effects of some of these factors can be
minimized through careful network design. For instance, a network can be
designed 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. Large internetworks,
however, can reduce the number of routers that must converge by using
static default routes for stub networks.
Although proper network design can
significantly reduce convergence time, a routing protocol's capability to
update and calculate routes efficiently can also improve convergence.
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