DUAL's sophisticated algorithm results in
EIGRP's exceptionally fast convergence. To better understand convergence -
using DUAL, consider the scenario in Figure . RTA can reach network 24
via three different routers: RTX, RTY, or RTZ.
In Figure , EIGRP's composite metric is
replaced by a link cost to simplify calculations. RTA's topology table
includes a list of all routes advertised by neighbors. For each network,
RTA keeps the real (computed) cost of getting to that network and also
keeps the advertised cost (reported distance) from its neighbor, as shown
in Figure .
At first, RTY is the successor to network
24, by virtue of its lowest computed cost. RTA's lowest calculated metric
to Network 24 is 31; this value is the FD to Network 24.
What if the successor to Network 24, RTY,
becomes unavailable, as shown Figure ?
RTA follows a three-step process to select
a feasible successor to become a successor for Network 24:
- Determine which neighbors have a
reported distance (RD) to Network 24 that is less than RTA's FD to
network 24. The FD is 31; RTX's RD is 30, and RTZ's RD is 220 (see Figure
). Thus, RTX's RD is below the current FD, while RTZ's RD is
not.
- Determine the minimum computed cost to
Network 24 from among the remaining routes available. The computed
cost via RTX is 40, while the computed cost via RTZ is 230. Thus, RTX
provides the lowest computed cost.
- 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
(the feasible successor) to forward packets to Network 24. The capability
to make an immediate switchover to a backup route is the key to EIGRP's
exceptionally fast convergence times. However, what happens if RTX also becomes
unavailable, as shown Figure ?
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 not less than RTA's FD of 31. Therefore, RTZ cannot
be a feasible successor (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 active state, the FD is
reset. This allows RTA to at last accept RTZ as the successor to Network
24.
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