Understanding EIGRP reported and feasible distance

[Tuff]

Hi!

  I need some help understanding EIGRP feasible distance calculation. To reach the 10.10.11.0 network from R4, the feasible distance is 5120. R4 reports its feasible distance to R3.... R3 has now a reported distance of 5120 to reach the 10.10.11.0 network  from R4.. So if i understand things correctly R3 will then calculate its feasible distance to reach the 10.10.11.0 network by calculating the metric between itself (R3) and R4 and it will add the reported distance from R4 to the calculation. 

Bandwidth between R3 and R4 is 1544kbits and the delay is 20000. So the metric between R3 and R4 is (10000000/1544 + 2000)256 = 2169 856. Which means that if i add the reported distance (5120) to 2169 856, i should get 2174 976 and that will be R3 feasible distance to reach the 10.10.11.0 network.

If that is the case, then can someone explain to me why i get different value than what is showing in packet tracer if i calculate the metric between R3 and R4 and i add the reported distance 5120 to it ? I dont understand why i dont get the same value as what is shown i packet tracer. What am i doing wrong or missing ?

Hi!

  I need some help understanding EIGRP feasible distance calculation. To reach the 10.10.11.0 network from R4, the feasible distance is 5120. R4 reports its feasible distance to R3.... R3 has now a reported distance of 5120 to reach the 10.10.11.0 network  from R4.. So if i understand things correctly R3 will then calculate its feasible distance to reach the 10.10.11.0 network by calculating the metric between itself (R3) and R4 and it will add the reported distance from R4 to the calculation. 

Bandwidth between R3 and R4 is 1544kbits and the delay is 20000. So the metric between R3 and R4 is (10000000/1544 + 2000)256 = 2169 856. Which means that if i add the reported distance (5120) to 2169 856, i should get 2174 976 and that will be R3 feasible distance to reach the 10.10.11.0 network.

If that is the case, then can someone explain to me why i get different value than what is showing in packet tracer if i calculate the metric between R3 and R4 and i add the reported distance 5120 to it ? I dont understand why i dont get the same value as what is shown i packet tracer. What am i doing wrong or missing ?

10.10.11.0 connected to R4 with feasible distanceR3 got a reported distance of 5120 to reach 10.10.11.0 from R4

[Jon Marshall]

You don't use the reported distance in the calculation which is where you are going wrong. 

You need to use the minimum bandwidth along the path which you have and then you need to add the delay of each outgoing interface to the destination which you haven't done ie. you have only used the delay on R3's connection to R4 but you also need to use the delay on R4's gi0/0 interface. 

If you add up all your delays and then repeat your calculation it should work out. 

Jon

[Tuff]

When i do the calculation without including the reported distance, i get the same result as shown in packet tracer. I got no problem with that. The confusion i am having according to what im learning is the reported distance is also included in the calculation. One of these articles is from ciscos website. This is what confuses me, so can you explain to me what they mean then if your saying the reported distance is not included in the calculation? I would appreciate it.

Feasible distance (FD) – the local router’s metric of the best route to reach a specific network. The metric is calculated using the metric reported by the neighbor (the advertised distance) plus the metric to the neighbor reporting the route. The route with the lowest FD will be placed in the routing table.

Feasible distance is the best metric along a path to a destination network, including the metric to the neighbor advertising that path. Reported distance is the total metric along a path to a destination network as advertised by an upstream neighbo

[Jon Marshall · Accepted solution]

The literature can be a bit confusing but as far as I understand it EIGRP does not send calculated values, it only sends the individual components used in the metric calculation eg. bandwidth, delay etc. 

So in your example R3 calculates the RD from R4 using the individual metric components advertised by R4. 

It also uses those components plus the metrics of the interface connecting to R4 to calculate the CD (Computed Distance) to the destination subnet.

Jon