EIGRP Metric Calculation Example - EIGRP Composite Metric Formula

EIGRP Metric = 256*((K1*Bandwidth) + (K2*Bandwidth)/(256-Load) + K3*Delay)*(K5/(Reliability + K4)))

Delay:Delay is a measure of the time it takes for a packet to traverse a route.
Load:reflects the amount of traffic utilizing the link. in this case a lower load value is more desirable because it indicates less load on the link.
Reliability:is a measure of the probability that the link will fail or how often the link has experienced errors, the higher the value, the more reliable the link.
Bandwidth: is a static value used by some routing protocols such as EIGRP and OSPF to calculate their routing metric.

By default, the values of K1 and K3 are set to 1, and K2, K4 and K5 are set to 0. Hence the above equation is deduced to

EIGRP Metric = 256*(Bandwidth + Delay)

EIGRP scales the interface values by inverting the bandwidth and scaling the delay with following calculations:.So, the Metric formula used by Eigrp is

Metric = 256*[10^7/BW(min) in Kbps] + [sum of delay in usec/10]

Wherein, BW(min) is the minimum bandwidth to reach a destination in Kilobits per seconds & Delay is in microseconds (usec)

Looking at the above diagram, we should be able to know the metric of a path from site 1 to site 3. Lets do the calculation.

Path Metric from Site1 to Site 3 through Site 2:

Metric = 256*[10^7/BW(min)] + [sum of delay]/10

Metric = 256*[10^7/1544] + [20000+20000+100]/10 Metric = 256*[6476 + 4010]

Metric = 2684416

Path Metric from Site1 to Site 3 through Site 4:

Metric = 256*[10^7/BW(min)] + [sum of delay]/10

Metric = 256*[10^7/100000] + [100+100+100]/10 Metric = 256*[100 + 30]

Metric = 33280

So, the path metric from Site1 to Site 3 through Site 4 is better than the path metric through site 3.So, Site1 router will use Site 3 through Site4 router.

Site1#sh ip route 

Codes: C - connected, S - static, I - IGRP, 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, E - EGP
       i - IS-IS, 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

     10.0.0.0/8 is variably subnetted, 7 subnets, 2 masks
C       10.10.10.0/24 is directly connected, FastEthernet0/0
C       10.10.11.0/30 is directly connected, Serial0/0/0
D       10.10.12.0/24 [90/2172416] via 10.10.11.2, 00:00:05, Serial0/0/0
D       10.10.13.0/30 [90/2174976] via 10.10.15.2, 00:00:14, FastEthernet0/1
D       10.10.14.0/24 [90/33280] via 10.10.15.2, 00:00:14, FastEthernet0/1
C       10.10.15.0/30 is directly connected, FastEthernet0/1
D       10.10.16.0/30 [90/30720] via 10.10.15.2, 00:00:14, FastEthernet0/1

Calculating Cost after changing the default values of bandwidth and delay, the configured bandwidth and delay values is shown in the picture below

Important Notes:

1. If you calculate the path metric from site 1 to site 3 network 10.10.14.0/24, you have to use the values (Bw & Delay) of the outbound interfaces, means the values of the f0/1 of site 1 router , and fa0/1 of site 4 router, and f0/0 of site 3 router.
2. If you calculate the path metric from site 3 to site 1 network 10.10.10.0/24, you have to use the values (Bw & Delay) of the outbound interfaces, means the values of the f0/1 of site 3 router, and fa0/0 of site 4 router, and f0/0 of site 1 router

Example:

I changed the default values of the bandwidth and delay for the interface f0/0 of site4 router, but all other routers has the default values of bandwidth and delay as following.

How to calculate the path metric

Path Metric from Site1 to Site 3= ?

Metric = 256*[10^7/BW(min)] + [sum of delay]/10

Metric = 256*[10^7/100000] + [100+100+100]/10

Metric = 256*[100 + 30]

Metric = 33280

Path Metric from Site3 to Site 1 = ?

Metric = 256*[10^7/BW(min)] + [sum of delay]/10

Metric = 256*[10^7/5] + [100+1200+100]/10

Metric = 256*[2000000 + 140]

Metric = 512035840

Site1#sh ip route

Codes: C - connected, S - static, I - IGRP, 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, E - EGP

       i - IS-IS, 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

     10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks

C       10.10.10.0/24 is directly connected, FastEthernet0/0

D       10.10.14.0/24 [90/33280] via 10.10.15.2, 00:05:24, FastEthernet0/1

C       10.10.15.0/30 is directly connected, FastEthernet0/1

D       10.10.16.0/30 [90/30720] via 10.10.15.2, 00:05:24, FastEthernet0/1

Site3#sh ip route

Codes: C - connected, S - static, I - IGRP, 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, E - EGP

       i - IS-IS, 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

     10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks

D       10.10.10.0/24 [90/512035840] via 10.10.16.1, 00:05:55, FastEthernet0/1

C       10.10.14.0/24 is directly connected, FastEthernet0/0

D       10.10.15.0/30 [90/512033280] via 10.10.16.1, 00:05:55, FastEthernet0/1

C       10.10.16.0/30 is directly connected, FastEthernet0/1

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