EIGRP Unequal Cost Load Balancing (maths)
the SG has
interface GigabitEthernet1.67 delay 25 ! interface GigabitEthernet1.146 delay 131 ! router eigrp 100 variance 5
This question's mathmatics baffles me. I have not done algebra since 1997.
I understand what needs to be done - but I do not underderstand the maths.
The solution guide goes from here (relavtive to this topic - dont worry about equal cost on R1)
The total delay of this path is 40 microseconds, or 4 tens of microseconds. Scaled by 256, R1 would be advertising 1024. Because R3's Feasible Distance of 1024 is equal to R6’s Feasible Distance, this path cannot be considered a Feasible Successor.
is this a typo?
Because the minimum configurable delay value is 10 microseconds, which is already the default for all Ethernet links, and based on task requirements, we need to modify R6's delay values on its VLAN 67 and VLAN 146 interfaces, so that metric through R1 is five times bigger than metric through R7.
then the formuale - which I do not know how to solve. - was the 250 arbitary?
5 * [Delay(Gi1.9) + Delay(Gi1.79) + Delay(Gi1.67)] = [Delay(Gi1.9) + Delay(Gi1.79) + Delay(Gi1.37) + Delay(Gi1.13) + Delay(Gi1.146)]. 5 * [10 + 10 +Delay(Gi1.67)] = [10 + 10 + 10 + 10 + Delay(Gi1.146)]].
I understand that the second line is a simplification of the first - but then how do you get the actual values for the delay on the interface? - It then suggests 250 - but I do not see the algebra workings:
If, for example, we configure delay on R6's VLAN 67 interface to be 250, in simple math we need to configure a delay value of 1310 on R6's VLAN 146 interface. This also means that configuring a variance of 5 will be enough so that both routes for VLAN 9 are installed in the routing table of R6 with the requested load distribution.
- was the 250 arbitary? - or does it have a direct correlation with the feasability condition - and if so how was it calculated? - I dont mean its obviously 25 x 10s of microseconds - I mean was this pulled out of a hat - could we have used. 500 and 2620 ?
250 + 20 = 270 * 5 which correlates to 1310 + 40 = 1350
But how was this worked out using maths to satisfy both the feasability condition and the 5 X load balancing ? guess work - or real algebra?