The best-route selection process

FIGURE 4 . 4 The best-route selection process
WAN
CO
Host X
172.7.8.0/24
Host Y
Host
RouterA
RouterB
RouterD
RouterC
20
30
172.3.4.4/30
172.1.2.4/30
172.6.7.4/30
172.5.6.4/30 172.10.10.0/24
172.11.12.4/30
35
35 20

Using RouterA as a starting point, we see that there are three different routes to Host Y. Each
link has been assigned a cost. In this example, RouterD and the WAN all have advertised costs to
Host Y that they send to RouterA. This is known as the advertised distance. To calculate the
advertised distance, add together the metrics between the advertising router and the destination
and multiply by 256. In order to determine the feasible distance, you need to add the metric to
reach a neighbor to the calculated metric from the advertising router to the destination and then
multiply by 256. The lowest calculated metric becomes the feasible distance, and that route
becomes the successor. Any route with advertised distance less than the feasible distance becomes
a feasible successor route.
Let’s calculate the lowest metric for Host X to get to Host Y. We’ll use the path from Host X
to RouterA to RouterB to RouterC and finally to Host Y for our first path calculation. To
calculate the total metric, we add 20 (RouterA to RouterB) to 30 (RouterB to RouterC) and
multiply it by 256 for a final value of 12,800.
Now let’s calculate the metric for the path from Host X to RouterA to the WAN to RouterD
and then to Host Y. Add the metric from the WAN to RouterD, 20, to the metric from the WAN
to RouterA, 35; the total is 55. Multiplying 55 by 256 gives us a value of 14,080.
Finally, we will calculate the total metric from Host X to RouterA to RouterD to Host Y.
Take the metric 35 and multiply it by 256 for a value of 8,960. The value 8,960 becomes the
feasible distance, and the path to RouterD becomes the successor.
In order to calculate the feasible successor routes, we need to look at the advertised distance
to the destination from each neighbor. The neighbors with an advertised distance lower than the
feasible distance will become feasible successors. In this case, all of the routes will become feasible
successors.
Information given in Table 4.3 closely, though not exactly, represents what is contained in
an actual topology table. 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 primary route.