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One of the most fascinating aspects of routing is a
concept known as convergence. Quite simply, whenever a change occurs in a
network's topology, or shape, all the routers in that network must develop a new
understanding of what the network's topology is. This process is both
collaborative and independent; the routers share information with each other,
but must independently calculate the impacts of the topology change on their own
routes. Because they must mutually develop an agreement of the new topology
independently from different perspectives, they are said to converge on
this consensus.
Convergence is necessary because routers are intelligent devices that can
make their own routing decisions. This is simultaneously a source of strength
and vulnerability. Under normal operating conditions, this independent and
distributed intelligence is a source of tremendous advantage. During changes in
the network's topology, the process of converging on a new consensus of the
network's shape may actually introduce instability and routing problems.
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