Actions and Factors Affecting EIGRP Route Selection

BD

The reported distance (or advertised distance) is the cost from the neighbor to the destination. It is calculated from the router advertising the route to the network.

For example in the topology below, suppose router A & B are exchanging their routing tables for the first time. Router B says Hey, the best metric (cost) from me to IOWA is 50 and the metric from you to IOWA is 90 and advertises it to router A. Router A considers the first metric (50) as the Advertised distance. The second metric (90), which is from NEVADA to IOWA (through IDAHO), is called the Feasible distance.

The reported distance is calculated in the same way of calculating the metric. By default (K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0), the metric is calculated as follows:

Feasible successor is the backup route. To be a feasible successor, the route must have an Advertised distance (AD) less than the Feasible distance (FD) of the current successor route.

Feasible distance (FD): The sum of the AD plus the cost between the local router and the next-hop router. The router must calculate the FD of all paths to choose the best path to put into the routing table.

Note: Although the new CCNA exam does not have EIGRP topic but you should learn the basic knowledge of this routing protocol.

The EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco proprietary routing protocol that uses advanced algorithms to calculate the best path to a destination network. The EIGRP route selection process takes into account several factors, including metrics and administrative distance, to determine the best path to a destination network.

Here are the two actions that influence the EIGRP route selection process:

1. The router calculates the feasible distance of all paths to the destination route.

Feasible distance (FD) is the total metric of a route, which is calculated by adding the advertised distance (AD) of the next-hop router and the metric of the outgoing interface. The AD is the metric that is assigned by the next-hop router to a particular route, and it is calculated based on the bandwidth and delay of the link. The metric of the outgoing interface is calculated based on the bandwidth and delay of the interface. The router calculates the FD of all paths to the destination route and selects the path with the lowest FD as the best path.

2. The router calculates the best backup path to the destination route and assigns it as the feasible successor.

A feasible successor (FS) is a backup path to a destination network that meets the feasibility condition. The feasibility condition states that the AD of the FS must be less than the FD of the current best path. The router calculates the FS for each path to the destination network and selects the path with the lowest AD as the FS. If the current best path fails, the router can immediately switch to the FS without the need for a new calculation.

Therefore, the correct answers are B and D. The router calculates the FD of all paths to the destination route, and it calculates the best backup path to the destination route and assigns it as the feasible successor.