EIGRP calculation

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Jun 17th, 2010
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Hello,


When calculating the Feasibility Distance between 2 routers how do I get the delay value, obviously I know the bandwidth as I can get this from the interface on router config, but I need the delay value so I can complete the FS calculation?


Many thanks

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Giuseppe Larosa Thu, 06/17/2010 - 06:18
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Hello Andy,


you need to sum all delays of all interfaces on the path (counted only one side per link in the direction to the network ) and to divide by ten.

The result needs to be multiplied by 256


256 * (sum of delays [tens of microseconds]) + 256 * 10000000 /BW*


BW* = min BW on path [ kbps]



sh interface provides you delay in microseconds, EIGRP formula uses tens of microseconds


example:



sh int f0/0

FastEthernet0/0 is up, line protocol is up

  Hardware is AmdFE, address is 0012.8056.ec00 (bia 0012.8056.ec00)

  Description: link XXXX

  Internet address is x.y.128.218/30

  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, => 10 tens of microseconds

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation ARPA, loopback not set

  Keepalive set (10 sec)



the bandwidth to be considered is the lowest on the path, not necessarily the bandwidth of the last interface




Hope to help

Giuseppe

Andy White Thu, 06/17/2010 - 07:00
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Thanks,


Between 2 routers the FD is [90/21024000] and both interfaces look like this (R1 to R2's WAN serial interfaces):


R1:

Serial0/1/0 is up, line protocol is up (connected)
  Hardware is HD64570
  Internet address is 200.0.0.2/30
  MTU 1500 bytes, BW 128 Kbit, DLY 20000 usec,


R2:

Serial0/1/0 is up, line protocol is up (connected)
  Hardware is HD64570
  Internet address is 200.0.0.1/30
  MTU 1500 bytes, BW 128 Kbit, DLY 20000 usec,


How do they get to the FD of 21024000?  I tried use the foumla but I get no where near this?

Andy White Thu, 06/17/2010 - 07:25
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Ah got it:


I did:


10^7/128 = 78125


(20000 + 20000)/10=4000


78125 = 4000 = 82125


82125 x 256 = 21024000



Andy White Fri, 06/18/2010 - 01:09
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How is the DLY calculated on an interface?  Is it a default value based on it's speed?


Thanks

Giuseppe Larosa Fri, 06/18/2010 - 05:28
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Hello Andy,


>> How is the DLY calculated on an interface?  Is it a default value based on it's speed?


yes it is an administrative parameter you can change it using delay command in interface mode


it does not mean it is the effective delay on the interface it is an administrative parameter


Hope to help

Giuseppe

Andy White Fri, 06/18/2010 - 05:36
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Thanks, so is there a table that shows these default delay values?

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