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Convergence in an EIGRP
network can be quite complicated. Part of this complexity is due to the
Cisco standard of supporting up to six parallel routes to all
destinations. Although this standard has helped make Cisco the market
leader in internetworking products, it makes it more difficult to explain
internetworking topics. For the sake of explaining the theory and
mechanics of convergence, the examples in this section assume that a
router remembers only a single primary route and one alternative (feasible
successor) route.
The network illustrated in Figure
is a relatively small EIGRP network with five separate
regions and a modest degree of route redundancy. This network is used to
illustrate how EIGRP converges using its DUAL algorithm and feasible
succession.
In this illustration, there are five different networks
within a single EIGRP AS. The routers are labeled A through E. Another
simplifying assumption in this example is that all the links are T1s.
Therefore, bandwidth and delay will be constant throughout the network,
and route selection becomes a less-confusing hop-counting exercise. So, instead of complicating the example with composite metrics, hop
counts are used.
Figure contains the known distances between router C and the other networks.
The Router C DUAL
algorithm has selected the least-cost paths from the multiple available
paths to networks 193.9.4 and 193.9.5. Figure
summarizes the router C view of the network. Note that none of these
routers have any feasible successors for any of these destinations because
the distance reported by the neighbor must be less than --- and NOT equal to --- the best metric available to reach that destination.
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