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| 5.4 |  | EIGRP Operation |
| 5.4.1 |  | Convergence using EIGRP |

The sophisticated algorithm of DUAL results in exceptionally fast EIGRP convergence. To better understand convergence when using DUAL, consider the scenario . RTA can reach network 24 via three different routers: RTX, RTY, or RTZ.

The composite metric of EIGRP is replaced by a link cost to simplify calculations. The RTA topology table includes a list of all routes advertised by neighbors. For each network, RTA keeps the real, computed, cost of getting to the network. RTA also keeps the advertised cost or the reported distance from its neighbor.

At first, RTY is the successor to Network 24, by virtue of its lowest computed cost. The lowest calculated metric from RTA to Network 24 is 31. This value is the FD to Network 24.

What if the successor to Network 24, RTY, becomes unavailable?

RTA follows a three-step process to select a feasible successor to become a successor for Network 24:

1. Determine which neighbors have reported distances (RD) to Network 24 that is less than the FD reported by RTA to Network 24. The FD is 31. The RD for RTX is 30, while the RD for RTZ is 220. Therefore, the RD for RTX is below the current FD, while the RD for RTZ is not.
2. Determine the minimum computed cost to Network 24 from among the remaining routes available. The computed cost by way of RTX is 40, while the computed cost by way of RTX is 230. Therefore, RTX provides the lowest computed cost.
3. Determine whether any routers that met the criterion in Step 1 also met the criterion in Step 2. RTX has done both, so it is the feasible successor.

With RTY down, RTA immediately uses RTX , which is the feasible successor, to forward packets to Network 24. The capability to make an immediate switchover to a backup route gives EIGRP exceptionally fast convergence time. However, what happens if RTX also becomes unavailable?

Can RTZ be a feasible successor? Using the same three-step process as before, RTA finds that RTZ is advertising a cost of 220, which is greater than the FD of 31for RTA. Therefore, RTZ cannot be a feasible successor as of yet. The FD can change only during an active-to-passive transition, and this did not occur, so it remains at 31. At this point, because there has not been a transition to active state for Network 24, DUAL has been performing what is called a local computation.

RTA cannot find any feasible successors, so it finally transitions from passive to active state for Network 24 and queries its neighbors about Network 24. This process is known as a diffusing computation. When Network 24 is in an active state, the FD is reset. This allows RTA to at last accept RTZ as the successor to Network 24.