EIGRP Metric

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EIGRP uses the following equation in computing its metric.

EIGRP metric = [K1 * bandwidth + (K2 * bandwidth)/256 – load) + K3 * delay]*[K5/(reliability + K4)]

By default, EIGRP’s K values are set to:

K1 and K3 = 1
K2, K4 and K5 = 0

Substituting K values yields to this condense equation.

EIGRP metric = [K1 * bandwidth + K3 * delay]
EIGRP metric = bandwidth + delay

where:

bandwidth is the minimum bandwidth (denoted by bandwidth(i)) along the path expressed in kilobits per second (Kbps).

bandwidth = 10s of microseconds scaled by 256
bandwidth = (10000000/bandwidth(i)) * 256

delay is the total of the delays configured on the interfaces along the path.

The delay (denoted by delay(i)) as shown in the show interface commands is in microseconds, so you must divide by 10 before you use it in the above formula.

delay = 10s of microseconds scabled by 256
delay = delay(i) * 256

Sample topolgy and metrics are taken from this post. For this example, ignore the path from R1 > R3 > R4.

Let’s derive how to get the EIGRP metric value of 2195456 (R4’s F0/0, network 24.0.0.0/24)

R1#sh ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is not set 34.0.0.0/24 is subnetted, 1 subnets D 34.0.0.0 [90/2221056] via 12.0.0.2, 00:07:26, Serial0/0 4.0.0.0/24 is subnetted, 1 subnets D 4.4.4.0 [90/2323456] via 12.0.0.2, 00:05:52, Serial0/0 24.0.0.0/24 is subnetted, 1 subnets D 24.0.0.0 [90/2195456] via 12.0.0.2, 00:07:24, Serial0/0 12.0.0.0/24 is subnetted, 1 subnets C 12.0.0.0 is directly connected, Serial0/0 13.0.0.0/24 is subnetted, 1 subnets C 13.0.0.0 is directly connected, Serial0/1

Firstly, let’s determine the lowest bandwidth in our network topology. The lowest bandwidth is 1544Kbps, the Serial0/0 link on R1 to R2. Next is to determine the accumulative delay, R1 > R2 > R4. Delays from R1 to R2 is 20000 microseconds and from R2 to R4 is 1000 microseconds.

Now that we have the required information, let’s put this in to the equation.

EIGRP metric = [K1 * bandwidth + K3 * delay]
EIGRP metric = (10000000/bandwidth(i)) * 256 + delay(i) * 256
EIGRP metric = [(10000000/bandwidth(i)) + delay(i)] * 256
EIGRP metric = [(10000000/1544) + (21000/10)] * 256
EIGRP metric = [6476.683937824** + 2100] * 256
EIGRP metric = [6476 + 2100] * 256
EIGRP metric = 2195456

**Ignore the decimal values.

Conclusion: Just memorize the condense equation rather than the original one. In real world design, default K values are mostly use and acceptable. I never encounter in any customer setup where the K values are change.

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