It is not necessary to know, but it is a good idea for me to understand how the EIGRP metric is derived.
Total outgoing minimum bandwidth = (K1 * bandwidth) + (K2 * bandwidth)
Where K1 = bandwidth and K2 = load.
Cumulative Load, Delay and Reliability calculation =[ (256 – load) + (K3 * delay)] * [K5 /(reliability + K4)]
Where K3 is the delay, K4 is reliability, K5 is a value that affects reliability, if K5 is 0 K4 will be insignificant, from Jeremy Cioara’s video K4 and K5 are reliability values but I cannot find proof from Cisco doc, the documentation about the metric I found is based on Enhanced Interior Gateway Routing Protocol – Cisco System. From someone’s book it said K5 is MTU, no proof as well. From the formula I can assure K4’s usefulness depends on K5.
IGRP = Total outgoing minimum bandwidth / (Cumulative of Load, Delay and Relability)
EIGRP = 256 * IGRP
As above, I have broken down the formula into two parts. The full formula is:
(K1 * bandwidth + K2 * bandwidth) / [(256 – load) + (K3 * delay)] * [K5 / (reliability + K4)]
Default K values
K1 = K3 = 1
K2 = K4 = K5 = 0
Makes the default formula to be (K1 * bandwidth) / (256 – load) + (K3 * delay)
Makes the IGRP metric calculation to be minimum bandwidth + sum of delay, hence EIGRP metric to be 256 * IGRP metric.
Default EIGRP metric = 256 * (10^7/min-BW + sumDly/10)
MTU in EIGRP metric
Metric is not in EIGRP formula, but EIGRP records the minimum MTU along the path.
1841-1#sh ip route 10.1.34.2
Routing entry for 10.1.34.0/30
Known via “eigrp 90”, distance 90, metric 41024000, type internal
Redistributing via eigrp 90
Last update from 10.1.24.1 on Serial0/1/0, 00:02:29 ago
Routing Descriptor Blocks:
* 10.1.24.1, from 10.1.24.1, 00:02:29 ago, via Serial0/1/0
Route metric is 41024000, traffic share count is 1
Total delay is 40000 microseconds, minimum bandwidth is 64 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
1841-1#sh ip protocol
Routing Protocol is “eigrp 90”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
Default IGRP metric: (10^7/64) + 40000 = 1602500
Hence EIGRP metric = 256 * 1602500 = 410240000