Only the routes through RouterC and RouterE are feasible successors on RouterA for the 172.20.0.0/24 network. Feasible successors are backup routes that can be used immediately if the successor route is lost? the successor route is the best route to a destination. To qualify as a feasible successor, the advertising router must be closer to the destination than the router to which the route is advertised. To ensure that the advertising router meets this feasibility condition, compare the advertised distance (AD) to the feasible distance (FD) of the successor. The AD, which is also called the reported distance (RD), is the cost that the nexthop router has calculated for the route, and the FD is the cost that the local router has calculated for the route. If the AD of a route is lower than the FD of the successor, the route is a feasible successor.
By default, the Enhanced Interior Gateway Routing Protocol (EIGRP) composite metric is calculated by bandwidth and delay. The route with the lowest metric is the best route to the destination. EIGRP uses the lowest bandwidth along the path to a destination to calculate the bandwidth portion of the metric. Higher bandwidth values create lower metric values. By contrast, EIGRP uses the sum of the delays along the path to a destination to calculate the delay portion of the metric. Lower delay values create lower metric values. Therefore, higher bandwidth values and lower delay values are preferred over lower bandwidth values and higher delay values.
The following table displays the FD values for each route from RouterA to the 172.20.0.0/24 network:
| Path | Bandwidth | Total Delay |
|---|
| A-B-E* | 100 | 30 |
| A-C-E | 100 | 40 |
| A-D-E | 80 | 40 |
| A-E | 100 | 50 |
The table shows that the route through RouterB has the highest bandwidth and lowest total delay. Therefore, the route through RouterB is the successor. For a route to qualify as a feasible successor, the metrics at the nexthop router should be lower than the metrics of the best route on the local router. The following table displays the AD values for each nexthop router to the 172.20.0.0/24 network:
| Nexthop | Bandwidth | Total Delay |
|---|
| RouterB | 100 | 20 |
| RouterC* | 100 | 20 |
| RouterD | 80 | 30 |
| RouterE* | 100 | 10 |
The route through RouterC is a feasible successor because the AD from RouterC is lower than the FD through RouterB. The bandwidth of the AD from RouterC is the same as the bandwidth of the FD through RouterB. However, because the total delay of the AD from RouterC is less than the total delay of the FD through RouterB, the route through RouterC is a feasible successor.
The route through RouterE is a feasible successor because the AD from RouterE is lower than the FD through RouterB. The bandwidth of the AD from RouterE is the same as the bandwidth of the FD through RouterB. However, because the total delay of the AD from RouterE is less than the total delay of the FD through RouterB, the route through RouterE is a feasible successor.
The route through RouterD is not a feasible successor, because the AD from RouterD is higher than the FD through RouterB. The total delay of the AD from RouterD is the same as the total delay of the FD through RouterB. However, because the bandwidth of the AD from RouterD is lower than the bandwidth of the FD through RouterB, the route through RouterD is not a feasible successor.