Eigrp metric calculation

[acbenny]

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

Load is 1/255

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

[jsayer · Accepted solution]

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 · Accepted solution]

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]

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.