Today while i was doing lab i was stuck in the scenario where i have to do unequal cost load balancing in the eigrp domain.After many hours i finally got the desired result , thought of sharing this , kindly tell me if i am wrong anywhere.
One of the most important factors in choosing EIGRP as an IGP in the Network is the ability of doing unequal cost load balancing.To enable this feature into Eigrp we need to issue “Variance” command under the Eigrp process.In order for path to be considered for unequal cost load balancing it must be a feasible successor with a metric less than or equal to the successor’s metric times the Variance.
Feasible Distance= Local Distance + Advertised Distance
A path whose advertised distance is lower than the feasible distance of the successor is deemed as feasible successor
Note:-Only routes that are feasible successors can be used for unequal cost load
The formula for Eigrp Metric calculation:-
Metric = [k1 * bandwidth + (k2 * bandwidth)/(256 – load) + k3 * delay] *[k5/(reliability + k4)]
The “k” values are derived from the metric weights command, where K1 and
K3 are 1 by default and all other values are 0. This essentially means that only
bandwidth and delay are taken into account. “Bandwidth” is the inverse
bandwidth in Kbps times 107 (107/BWKbps). “Delay” is delay in tens of microseconds (DLYusec/10). These values are added together and then scaled
by a factor of 256. The composite metric is therefore represented by default as:
Metric = (107/BWKbps + DLYusec/10) * 256
Note:-Try to use Delay instead of Bandwidth because by changing bandwidth and QoS is running in the network , it will effect the traffic drastically in the network.
In order to achieve the any desired ratio suppose we want to distribute traffic in a ratio of 3:1 among the successors and feasible successors, and we are using delay we can set our equation like BW1 and Delay1 for the one path and BW2 and Delay 2 for other path.Now the equation will look like this.
3*[107/BW1 in Kbps + Delay1 in usec/10) * 256]=(107/BW2 in Kbps + Delay2 in usec/10) * 256
Here in the above formula watchout for BW1 ,Delay1,BW2 and Delay2.
By taking BW1 and BW2 as it is from the interface , we can find the end result something like :- Delay1=something times Delay2 ,now we can put the value of Delay 2 to find out Delay 1.
Lastly we can apply this Delay to the interface and check and verify our configuration by :-
sh ip route [destination network address] | include frame|share
Check the ratio in the traffic share count to see the exact result between both the paths.Also dont forget to include the Variance command in the Eigrp process.
sh ip eigrp topology [destination network address]
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