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Monday, August 9, 2010

Explanation of EIGRP Metric Calculation

P, 1 successors, FD is 284160
via (284160/281600), FastEthernet2/0
via (307200/281600), FastEthernet0/1

Why is the FD = RD + 256*Delay, but not FD = RD + 256*Delay + 256*(10000000/BW)?
Does this mean that on the local router we can play only with delay to influence the metric?

EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

  • Only the lowest BW on path is used for the EIGRP metric calculation and indeed the received update contains in separate fields all the metric components with their value from the point of view of their neighbor (RD).
  • If the interface to neighbor bandwitdth is greater or equal to the lowest bandwidth in path that part of the EIGRP metric does not change.
  • As a result of this the safer way to change EIGRP metric is to play with delay as you have correctly noted, changing the bandwidth does not provide a guarantee to change the total metric calculation.
  • Delay is the sum of all delays on path.
  • Cumulative in delay and inverse proportional to lowest bandwidth in path this is EIGRP metric.

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