EIGRP FS calculation

[catalystexpress]

Hi All,

Can you all please help me to understand a few concepts based on below toplogy

       R1 ----------172.17.70.0/30------------- R2

         |                                                |

172.17.70.8/30                   172.17.70.4/30

          |                                                |

  ------------------------------------------------------------

                         R3

have configured loopback's in all the routers and included the same in EIGRP network, as expected there is one sucessor and two learned FD from neigbhor router as below, all the fa interface are 10Mbs link and DLY is 1000usec...

my objective is to user eigrp, and play around the metric and make sure there is a FS always..

R1#sh ip eigrp topology all-links

IP-EIGRP Topology Table for AS(100)/ID(10.3.33.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

       r - reply Status, s - sia Status

P 10.3.3.0/24, 1 successors, FD is 128256, serno 1

        via Connected, Loopback100

P 10.4.3.0/24, 1 successors, FD is 156160, serno 25

        via 172.17.70.9 (409600/128256), FastEthernet0/1

        via 172.17.70.2 (435200/409600), FastEthernet0/0

P 10.6.3.0/24, 1 successors, FD is 409600, serno 22

        via 172.17.70.2 (409600/128256), FastEthernet0/0

        via 172.17.70.9 (435200/409600), FastEthernet0/1

P 10.4.31.0/24, 1 successors, FD is 156160, serno 26

        via 172.17.70.9 (409600/128256), FastEthernet0/1

        via 172.17.70.2 (435200/409600), FastEthernet0/0

P 10.6.31.0/24, 1 successors, FD is 409600, serno 23

        via 172.17.70.2 (409600/128256), FastEthernet0/0

        via 172.17.70.9 (435200/409600), FastEthernet0/1

P 10.3.31.0/24, 1 successors, FD is 128256, serno 2

        via Connected, Loopback101

P 10.3.33.0/24, 1 successors, FD is 128256, serno 4

        via Connected, Loopback103

P 10.3.32.0/24, 1 successors, FD is 128256, serno 3

        via Connected, Loopback102

P 10.6.32.0/24, 1 successors, FD is 409600, serno 24

        via 172.17.70.2 (409600/128256), FastEthernet0/0

        via 172.17.70.9 (435200/409600), FastEthernet0/1

P 10.4.32.0/24, 1 successors, FD is 156160, serno 27

        via 172.17.70.9 (409600/128256), FastEthernet0/1

        via 172.17.70.2 (435200/409600), FastEthernet0/0

P 172.17.70.8/30, 1 successors, FD is 281600, serno 16

        via Connected, FastEthernet0/1

P 172.17.70.0/30, 1 successors, FD is 281600, serno 15

        via Connected, FastEthernet0/0

P 172.17.70.4/30, 2 successors, FD is 307200, serno 21

        via 172.17.70.2 (307200/281600), FastEthernet0/0

        via 172.17.70.9 (307200/281600), FastEthernet0/1

R1#

if you look at the router R1 capture for the network 10.4.3.0/24

we can notice the FD is 156160

however In "show ip eigrp toplogy" the FD is 409600 --> blieve this is the local metric

and the nbr advertised metric is 128256 for the same network

so how come the "show ip eigrp toplogy all-links" show the FD as 156160.........

Also am not understanding how the metric calculation is happening in this topology

for the loopback am not sure how to calculate...i think the delay is 500 ...

can anyone please explain this for me

many thanks to all for sharing the knowledge........

[Peter Paluch · Accepted solution]

Hello,

The feasible distance (FD) is not the current metric to the destination - this is one of the most misunderstood things about the EIGRP. Rather, the FD is the minimal distance to the distance since the last time the route became passive. In a sense, it is the historical minimum of the distance to the destination network, with the history beginning anew after the route comes from Active to Passive state.

In your particular case, the 10.4.3.0/24 network is described as follows:

P 10.4.3.0/24, 1 successors, FD is 156160, serno 25

        via 172.17.70.9 (409600/128256), FastEthernet0/1

        via 172.17.70.2 (435200/409600), FastEthernet0/0

The FD is 156160 which means that the router once was as close as 156160 to the 10.4.3.0/24. However, the distance to the destination has increased since that time although the route did not need to become Active. Therefore, the FD stayed at its current value while the real distance has increased. The current distance is 409600 via 172.17.70.9, which is the current successor. You do not have a feasible successor because the 172.17.70.2 does not meet the feasibility condition: its reported distance of 409600 is not strictly lower than your FD of 156160. In other words, the neighbor is not closer to the destination than we have ever been since the last Active->Passive transition, and so it is not trustworthy enough of becoming a feasible successor.

The FD is used only for the feasibility condition check, and is never advertised to another EIGRP routers. It is a local variable assigned to a particular destination, used within the feasibility condition, but it is independent of the actual distance to the destination - it may be equal if the current distance has not increased since the last Active->Passive transition, but it also may be lower if the current distance has increased (for example by increasing the Delay of a directly connected interface) and the router still had usable successors or feasible successors.

Best regards,

Peter

[catalystexpress]

Thank you Peter , i got the concept now...this should help me...thanks again...cheers...

[cadet alain]

Hi,

The FD of 156160 is correct but it should be the same under not 409600.

Clear your neighbour adjacencies and everything should be synced and correct.

For the loopback the delay is 5000 us so 500 tens of us, you're correct.

the FD is calculated this way: 256((10 to power 7)/10000000 + 500) =128256

Regards.

Alain.

[catalystexpress]

Thanks a lot Alain,  actually as peter explained the FD 156160, is router once was as close as 156160 to the 10.4.3.0/24. However, the  distance to the destination has increased since that time although the  route did not need to become Active. Therefore, the FD stayed at its  current value while the real distance has increased.

Now after i reset the EIGRP its showing up as 409600...which is correct...thanks a lot ....cheers....

[cadet alain]

Hi,

I didn't know you had modified delays or had a route coming active then passive.

As Peter explained:

it may be equal if the current distance has not increased since the last Active->Passive transition

I had a stable topology when testing  so my answer.

Regards.

Alain.

[catalystexpress]

sorry alain....i had not modfied anything, but i guess Peter guessed it rightly.....eitherway am still sorry.....and am very thankful that you did bother to share your knowledge ...which is great...thanks again for the support and lookforward for more from you....cheers....