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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
![]() The composite metric of EIGRP is replaced by a link cost to
simplify calculations.
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:
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.
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