# Eigrp metric calculation

Feb 18th, 2007

Hi expert,

As showed below, the FD(metric) is 5511936 and I try to calculate it by formula and get "10120000" . What's wrong ? why they are different ?

2620#sh ip eigrp topology 172.20.1.4 255.255.255.252

IP-EIGRP (AS 1): Topology entry for 172.20.1.4/30

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 5511936

Routing Descriptor Blocks:

0.0.0.0 (Serial0/0), from Connected, Send flag is 0x0

Composite metric is (5511936/0), Route is Internal

Vector metric:

Minimum bandwidth is 512 Kbit

Total delay is 20000 microseconds

Reliability is 255/255

Minimum MTU is 1500

Hop count is 0

FD(Metric) =[10000000 / bw (kbps) + delay(microsec) ] *256

FD(Metric) = [10000000 / 512 + 20000 ] *256

FD(Metric) = 39531.25 * 256 = 10120000

Hello Jsayer,

FD = 256 * ((10000000/512) + (20000/10))

= 256 * (19531.25 + 2000)

= 256 * 21531

= 5511936

Cisco IOS? rounded bandwidth calculation to 19531 so it give's exact mentioned FD.

The bandwidth term is correct, but remember that the delay value in the formula is expressed in 10s of microseconds. So the correct value is [10000000 / 512 + 2000 ] *256 = 5512000. Pretty close to the value the router calculated.

I've always assumed the slight variation that you get is due to 1kbps=1024 bits rather than 1000 bits, but I don't know if I've ever validated that theory.

Hope this helps,

John Sayer

Overall Rating: 5 (2 ratings)

## Replies

jsayer Sun, 02/18/2007 - 08:18

The bandwidth term is correct, but remember that the delay value in the formula is expressed in 10s of microseconds. So the correct value is [10000000 / 512 + 2000 ] *256 = 5512000. Pretty close to the value the router calculated.

I've always assumed the slight variation that you get is due to 1kbps=1024 bits rather than 1000 bits, but I don't know if I've ever validated that theory.

Hope this helps,

John Sayer

wpharaon Sun, 02/18/2007 - 09:08

Hello Jsayer,

FD = 256 * ((10000000/512) + (20000/10))

= 256 * (19531.25 + 2000)

= 256 * 21531

= 5511936

Cisco IOS? rounded bandwidth calculation to 19531 so it give's exact mentioned FD.

mrinmoy.m Mon, 02/19/2007 - 05:00

See...here the calculation shows that you are calculating the advertised distance

but the FD = Advertised Distance + the cost to reach the advertising neighbour

FD(Metric) = [10000000 / 512 + 20000 ] *256

FD(Metric) = 39531.25 * 256 = 10120000

This is not the right way. This should be rather...

10000000/512 + 20000/10*256

this will give u a diatance of 5512000

Regards

Kunal P.