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Chapter 6
total cost, we add 20 (RouterA to RouterB) to 30 (RouterB to RouterC), for
a final value of 50. For the feasible successor calculation, RouterA tells
RouterB the cost of 35, which is the advertised cost. B then adds its cost to
get to RouterA. This becomes 35 + 20, for a total path cost of 55.
The next path calculated is from Host X to RouterA to RouterD to Host
Y. In this case, there is no advertised cost, so the final value consists of only
the feasible cost of 35. The final path is calculated in the same manner to give
us the result of 55.
Since the lowest cost was 35, the route to learned via
RouterD will be chosen as the successor or primary route. The other two
routes remain in the topology table as feasible successors and are used if the
successor to Host Y fails.
Information given in Table 6.4 closely represents what is contained in an
actual topology table, though not exactly. The Status field shows whether a
new route is being calculated or if a primary route has been selected. In our
example, the route is in passive state because it has already selected the pri-
mary route.
The route with the best metric contains the lowest metric value and is cho-
sen as the primary route. If there is more than one route to a destination, the
route with the second-lowest metric will be chosen as the feasible successor,
as long as the advertised distance of the potential feasible successor is not
greater than the distance of the successor. Primary routes are moved to the
routing table after selection. More than one route can be made a primary
route in order to load balance. This will be discussed in the "Load Balanc-
ing" section later in this chapter.
T A B L E 6 . 3
Topology Table Information
Route--Adjacent Router's
Address (Metrics)
Number of
(3611648/3609600) via
(4121600/3609600) via
(Router C)
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