bandwith metric in eigrp

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Jan 31st, 2008
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Given the output from "show ip route 10.1.0.0" command; what is being done to the specified minimum bandwidth to arrive at the metric? I know the formula comes down to 256(bw+delay) but bw is in Kbps and ends up being divided into either 10E6 or 10^7th...I can't recall which it is. For the first route I can easily derrive the number the router gets for a metric if I use this:

256(100000000/10000 + 21000). But for the second route I can't get the number they have no matter what I do. These are off my actual switch. Let me start by asking clarification on what is the divisor when plugging in bandwidth, which the metrics specify in Kbps?


Routing Descriptor Blocks:

* 10.64.1.3

Route metric is 793600, traffic share count is 120

Total delay is 21000 microseconds, minimum bandwidth is 10000 Kbit

Reliability 255/255, minimum MTU 1500 bytes

Loading 1/255, Hops 1

Route tag 65000

10.64.1.2,

Route metric is 1070592, traffic share count is 89

Total delay is 25000 microseconds, minimum bandwidth is 5945 Kbit

Reliability 255/255, minimum MTU 1500 bytes

Loading 1/255, Hops 1

Route tag 1803

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robert.horrigan Thu, 01/31/2008 - 11:46
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EIGRP Metric = 256*([K1*Bw + K2*Bw/(256-Load) + K3*Delay]*[K5/(Reliability + K4)])




Edison Ortiz Thu, 01/31/2008 - 11:55
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The divisor is 10^7 and it's rather odd on the second Route Metric.


I was able to duplicate it by plugging the bandwidth & delay here.


Based on my calculation, the correct metric is 1070613 so we have a 21 point discrepancy.


__


Edison.

suelange Thu, 01/31/2008 - 12:18
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Thank you. I must be the biggest brick in the world When I plug this in for the second route, I'm not 21 points off...I'm way off. Using default metric weights:


metric = 256(10^7/bw + delay)


Given bw=5945 and delay is 25000, as per the second route, here is what I get. What am I doing wrong? 10^7 is 1 and seven zeros, right? Geez I feel like I have a box on my brain.


10000000/5945 = 1682.0857863751051303616484440706

+25000=26682.085786375105130361648444071

*256=6830613.9613120269133725820016821


I don't care how it gets rounded, 6830613 is nothing near 1070592, which is the metric shown....


Only thing that comes close is to use 10^8 not 10^7, then I come up with the right number but it has an extra zero at the end, which is WAY off...(that is to say if I use 10^8 I get 10705920).


I hate to ask simple math problems on this forum but I can't pass CCNP if I can't reliably trust myself to work the formula.

Istvan_Rabai Thu, 01/31/2008 - 12:30
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Hi Sue,


Your calculations are OK except one thing:


You should use the total delay/10.

The formula uses 10s of microseconds, not microseconds.


The you will get the same result as Edison.


Cheers:

Istvan


suelange Thu, 01/31/2008 - 12:30
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DOH!!!!!


Thanks! :-)

Istvan_Rabai Thu, 01/31/2008 - 12:36
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By the way,


EIGRP, when calculating the 10.000.000/bandwidth, makes the result an integer value.


This is why Edison has 21 points of discrepancy.


Cheers: Istvan

Edison Ortiz Thu, 01/31/2008 - 12:47
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Istvan,


Good catch. Integer value it is.


10^7/5945=1682 (integer value)

1682+2500=4182


256*4182=1070592



Istvan_Rabai Sun, 02/03/2008 - 12:50
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Thanks Edison,


I'm glad I could contribute to this thread.


Cheers:

Istvan

Edison Ortiz Thu, 01/31/2008 - 12:36
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I see where your problem is, is with the delay.


From

http://www.cisco.com/warp/public/103/eigrp-toc.html



EIGRP uses the following formula to scale the delay:


*


delay = delay(i) * 256


where delay(i) is the sum of the delays configured on the interfaces, on the route to the destination network, in tens of microseconds. The delay as shown in the show ip eigrp topology or show interface commands is in microseconds, so you must divide by 10 before you use it in this formula. Throughout this paper, we use delay as it is configured and shown on the interface.



__


Edison.

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