Ive hated this formula and avoided it for about 5 years. I just buckled down and figured it out, and actually its 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 thats 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. Weve only got 2 values to work out, so lets 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