EIGRP AD RD & FD

[Pradeep H A]

Hi All,

While studying about EIGRP I found this diagram online & was confused by the description of how EIGRP chooses its best route...

Can some one please tell from the diagram, what would be the Advertised Distance, Reported Distance & Feasible Distance for the network 172.30.1.0/24 at R5 from each of R2, R3, & R4. Who would/wouldn't be the successors & feasible successors & why?

Thanks a lot in advance

[Pradeep H A]

Just in case if the diagram is not clear:

R1 to R2 : BW=90 ; Delay=10

R1 to R3 : BW=100 ; Delay=10

R1 to R4 : BW=80 ; Delay=10

R2 to R5 : BW=100 ; Delay=10

R3 to R5 : BW=100 ; Delay=10

R4 to R5 : BW=100 ; Delay=10

[Peter Paluch]

Hello Pradeep,

Remember the following definitions:

1. Feasible Distance is the lowest known distance to the destination since the last transition of that destination from Active to Passive state. Feasible Distance may therefore be equal to the current lowest distance to the destination, or, in certain circumstances, it may also be lower than the current distance.
2. Reported Distance is the distance advertised (reported) by a neighbor. The terms Advertised Distance and Reported Distance are identical and they both describe the same thing. It is, however, recommended not to use the term Advertised Distance because its acronym AD get easily confused with administrative distance which is a totally unrelated concept.

So for R5, the distances would be:

• Reported from R2: BW=90, Delay=20
• Reported from R3: BW=100, Delay=20
• Reported from R4: BW=80, Delay=20

Total distances must also take the links between R5 and R2/R3/R4 into account, i.e.:

• Total via R2: BW=90, Delay=30
• Total via R3: BW=100, Delay=30
• Total via R4: BW=80, Delay=30

The lowest total distance of R5 towards 172.30.1.0/24 will be through R3 which will initially also become the Feasible Distance.

Best regards,

Peter

[Pradeep H A]

Hi Peter,

Thanks for the reply. But If i have 3 different routes like this:

BW=90 Delay=30

BW=80 Delay=20

BW=70 Delay=10

which one is better? & why?

[Peter Paluch · Accepted solution]

Hello Pradeep,

In order to answer this question, you have to compute the resulting EIGRP metric from these component metrics, and compare the resulting numbers.

The formula to compute EIGRP metrics from individual components in default configuration is:

Metric = [ (10^7 DIV minBW) + Delay ] * 256

where DIV is an integer division (perform the normal division and take the integral part, throwing the fraction away).

So let's see:

• [ (10^7 DIV 90) + 30 ] * 256 = 28452096
• [ (10^7 DIV 80) + 20 ] * 256 = 32005120
• [ (10^7 DIV 70) + 10 ] * 256 = 36573952

Obviously, the path with BW=90, Delay=30 wins.

Best regards,

Peter