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Classic EIGRP Metric Calculation

I’ve hated this formula and avoided it for about 5 years. I just buckled down and figured it out, and actually it’s pretty easy providing we are using default K values K1=1 and K3=1 (i.e. BW and Delay,) are used for the metric. The entire metric formula can actually just become BW + Delay. Bare with me, see the math below:

 

The actual formula is below

Metric=[K1* BW + (K2*BW)/(256*load) + K3 * Delay]

Since K2 = 0, this means the whole line of (K2*BW)/(256*load) can be completely removed (because 0 divided by anything is 0). So the formula becomes

Metric= K1*BW + K3*Delay

Since K1=1 and K3=1, the formula can be simplified again to

Metric= BW + Delay

 

So that’s it. Although there is a catch, which is how all the exam questions always try and catch you out. In the EIGRP metric calculation, the BW is not actually the BW & the delay is not actually the delay. How annoying is that? See below

  • Bandwidth = Inverse lowest bandwidth along a path in Kbps x 10^7 * 256.
  • Delay = Lowest cumulative delay along the path x 256

My first question was, what the heck is inverse bandwidth? Actually it just means 1/BW.  Let do a real life example of the calculation then. Take a look at the show output below, we will calculate how EIGRP came up with the metric as being 28160 for this network.

UKINTR1#sh ip eigrp topology 200.70.80.0/24
EIGRP-IPv4 Topology Entry for AS(100)/ID(10.66.255.1) for 200.70.80.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 28160
  Descriptor Blocks:
  0.0.0.0 (GigabitEthernet0/1), from Connected, Send flag is 0x0
      Composite metric is (28160/0), route is Internal
      Vector metric:
        Minimum bandwidth is 100000 Kbit
        Total delay is 100 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 0
        Originating router is 10.66.255.1

So we simplified the metric formula to BW + Delay. We’ve only got 2 values to work out, so let’s start by working out the bandwidth.

Bandwidth = Inverse lowest bandwidth along a path in Kbps x 10^7 x 256.
Bandwidth = 1/100000 x 10^7 x 256
Bandwidth = 25600

Now lets work out the delay

Delay = Lowest cumulative delay along the path x 256
Delay = 10 x 256 (remember delay is in “Tens of Microseconds”, whereas the output above shows only microseconds).
Delay = 2560

Now lets put this into the Metric

Metric = BW + Delay
Metric = 25600 + 2560
Metric = 28160

2 Comments

OrlaJune 12th, 2015 at 9:05 pm

Thanks that’s a very easy to understand explanation 🙂

AnonymousSeptember 12th, 2016 at 3:20 pm

Good Explanation

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