6.4 Convergence Using EIGRP
6.4.1 DUAL algorithm and feasible succession
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.