6 Replies Latest reply: May 16, 2012 1:36 AM by AshwinR

# EIGRP metric calculation ???

I am a newbie to this site ,

Plz find the attachment file below

the problem is routing table (if i want to reach R4 from R1 i have 2 paths )

R1#sh ip route

192.168.4.0/30 is subnetted, 1 subnets

D       192.168.4.0 [90/20514560] via 192.168.1.2, 00:46:48, Serial1/0

i know that (90) is internal ad of eigrp and (20514560) is the "successor" route i got it by calculation and its fine.

R1#sh ip eigrp topology

P 192.168.4.0/30, 1 successors, FD is 20514560

via 192.168.1.2 (20514560/28160), Serial1/0

i know the first value (20514560) is feasilble successor and i calculated it and its fine

WHERE I LACKING IS WHAT IS (28160) value refer to be and how can i calculate to get it .... thk u ... any help much appreciated

• ###### 1. Re: EIGRP metric calculation ???

The second number is your neighbor's (R2) metric/cost to reach the route (192.168.4.0)

• ###### 2. Re: EIGRP metric calculation ???

plz give me brief summay for this thank you much appreciated.....

• ###### 3. Re: EIGRP metric calculation ???

Ø  Feasible Distance  = How far is it from the current router to reach a specific route/destination.

Ø  Advertised Distance  = How far away is the neighboring router to that route/destination

So looking at (20514560/28160):

The 1stnumber is the Feasible Distance(FD), the 2nd number is the Advertised Distance (FD)

To sum it up:

AD = Distance from the neighboring router to the destination network.

FD = AD + The cost between your router and a neighbor router.

So if Router A and B are connected and there is a network/destination behind router B:

100                              10

Router A----------------------Router B--------------------| Network 192.168.4.0/30

The AD on Router A is 100, but the FD is 110. So Router B gets a a distance to network 192.168.4.0/30 advertise of 10, from Router B.

It will add the cost of the link between itself and Router B (100) to get the Feasible Distance.

• ###### 4. Re: EIGRP metric calculation ???

AshwinR,

Regarding the following statement:

"FD = AD + The cost between your router and a neighbor router.

So if Router A and B are connected and there is a network/destination behind router B:

100                              10

Router A----------------------Router B--------------------| Network 192.168.4.0/30

The AD on Router A is 100, but the FD is 110. So Router B gets a a distance to network 192.168.4.0/30 advertise of 10, from Router B.

It will add the cost of the link between itself and Router B (100) to get the Feasible Distance."

This is technically incorrect.  In fact, I wrote a little paper on the subject.  You can verify the results for yourself byu following these simple steps.

1) calculatie the cost from R2 to reach the network 192.168.4.0 using the EIGRP formula.  (this will be your AD or RD)

2) calculate the cost from R1 to reach R2 using the EIGRP formula. (this will be the cost to the next-hop router)

3) add the values obtained in #1 and #2. (this is the sum  --> FD = AD + cost to nex-hop)

4) calculate the FD from R1 to reach the network 192.168.4.0 using the EIGRP formula.  (this is the true FD to the destination)

5) compare the value obtained in Step #3 with that in Step #4.

The two values are not equal. Therefore, it is an inaccurate statement.

As I stated in my paper, the often quoted statement found in many Cisco Press books,

“The lowest-cost route is calculated by adding the cost between the next-hop router and the destination (referred to as the advertised distance [AD]) to the cost between the local router and the next-hop router. (The total is referred to as the feasible distance [FD].)”

is just plan incorrect.  This causes much confusion for beginners and even some CCIEs still get it wrong.  Read over my document titled, "EIGRP Metric Calculation Demystified" and hopefully it helps eliminate the confusion.  I hope you enjoy the paper.

https://learningnetwork.cisco.com/docs/DOC-13825

Hope this helps.

Brian

• ###### 5. Re: EIGRP metric calculation ???

Thank you Brian And AswinR

• ###### 6. Re: EIGRP metric calculation ???

Brian, I haven't fogotten about you. Just have to read your story twice and maybe more, to let it sink in...