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How to Calculate EIGRP Metric

How to Calculate EIGRP Metric

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EIGRP automatically calculates the routing table metric to choose the best path. But it is important that the network administrator understands how to calculate EIGRP metric. The default formula for the composite metric is:

  • [k1 x Bandwidth + k3 x delay] x 256
  • By default K1 and K3 is equal to 1 so the formula become
  • [Bandwidth + Delay] x 256

After examining the bandwidth and delay values for all of the outgoing interfaces of the route, the steps to calculate EIGRP metric are the following:

  1. Find out the link with the slowest bandwidth and use that value to calculate bandwidth using the formula 10,000,000/bandwidth.
  2. Find out the delay value for all outgoing interfaces on the way to the destination. Sum up the delay values of all interfaces and divide by 10.
  3. The sum of bandwidth and delay metric produce 24-bit value, but the EIGRP uses 32-bit value; therefore we multiply the 24-bit value with 256 to get a 32-bit value.

Calculating EIGRP Metric

Here in this example, we will use the same topology as used in earlier articles. This example displays how EIGRP determines the metric displayed in R1’s routing table for the 192.168.2.0/24 network.

Calculate EIGRP Metric

Bandwidth

We have earlier discussed that EIGRP uses the slowest bandwidth to calculate the metric. We can find out the slowest bandwidth by examining each interface between the source and destination. So, in this example, the bandwidth of the interface between the source router R2 and the destination network 192.168.2.0/24 is illustrated in figure-2.

The GigabitEthernet 0/1 interface on R1 has a bandwidth of 10000 kb/s. The GigabitEthernet 0/2 interfaces on R3 has a bandwidth of 1,000,000 kb/s. The bandwidth of the GigabitEthernet 0/1 interface on R1 is not the default bandwidth, because we have configured the bandwidth of this interface. So, the slowest bandwidth is 10000 kb/s and this bandwidth is used in the calculation of the metric.

The reference bandwidth of EIGRP is 10,000,000; the EIGRP divides this value by the interface bandwidth value in kb/s. The higher bandwidth gives lower metric value and the lower bandwidth produces higher metric value. So, 10,000,000 is divided by 10000 which results in 1000. If the result is not a whole number, then the value is rounded down.

Delay

The same outgoing interfaces are used to find out the delay value, as shown in Figure 1. We know that the EIGRP uses the sum of all delays along the path from the source to the destination. The GigabitEthernet interface on R1 has a delay value of 10 usec. The GigabitEthernet 0/2 on R3 has a delay value of 100 usec. The sum of these delays is 110, which is divided by 10 (110/10 = 11).

Calculate EIGRP Metric

Now we have calculated values for bandwidth and delay, so now we can calculate EIGRP metric. The simple formula for metric is [Bandwidth + Delay] x 256. To get the metric value we put the calculated values of bandwidth and delay in this formula for example [1000+11] x 256 = 258816. Now we can verify the calculated metric in the routing table for network 192.168.2.0/24. Figure 3 illustrates the output of the “show ip route” command.

EIGRP Composite Metric – Exclusive Explanation

EIGRP Composite Metric – Exclusive Explanation

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A composite metric is a number calculated based on several different components that determine the preferred route. By default, EIGRP uses bandwidth and delay to calculate the preferred path to a network. The reliability and load can also be used, but are not recommended, because they typically result in a frequent recalculation of the topology table.

  • Bandwidth: The bandwidth value used in the EIGRP metric is determined by dividing 10,000,000 by the bandwidth (in kbps) of the slowest outgoing interface along the path from the source to the destination network.
  • Delay: It is the cumulative value of all delays associated with all of the interfaces along the path from the source to the destination (in tens of microseconds). We can display the delay in the output of the show interfaces command.
  • Reliability: It is the worst reliability between the source and destination based on keep alive.
  • Load – Represents the worst load on a link between the source and destination, determined based on the packet rate and the configured bandwidth of the interface.

Figure 1 illustrates the EIGRP composite metric formula. The formula consists of K1 to K5 values, known as EIGRP metric weights. K1 bandwidth, K3 represent delay, K2 represents load, and K4 and K5 represent reliability. The bandwidth and delay values are used in the default composite metric. EIGRP for IPv4 and EIGRP for IPv6 both use the same formula for the composite metric. K1 and K3 values are set to 1 in the default formula, K2, K4 and K5 are set to 0.

EIGRP Composite Metric

The metric calculation technique (k values) and the autonomous system number of the EIGRP must match between neighbors; otherwise, routers do not perform an adjacency. The command for changing the default k values is “metric weights” router configuration mode command. The command syntax is as under:

Router(config-router)# metric weights <tos k1 k2 k3 k4 k5>

TOS stands for the Type of Service Byte in an IPv4 packet header. It is intended to be used for a quality of service marking. But, the value of TOS must be 0. The five k values can be set into the range between 0 – 255. The metric weights value must be the same for establishing neighbor adjacencies. If metric weights values are different, then adjacency does not form.

Verifying the k Values

We can verify the k values using the “show ip protocols” command. Figure 2 illustrates the output of this command for R1. The k values on R1 are set to the default.

K Values

Examining Interface Values

We can examine the values used in the metric calculation using the “show interfaces” command. The command also displays complete interface information. Figure 3 illustrates the show interfaces command output for the Serial 0/0/0 interface on R1. The parameters used in the metric calculation formula are marked with a red rectangle.

  • BW– This is the interface bandwidth. The bandwidth is in kilobits per second.
  • DLY– This is the delay of the interface. The delay is in microseconds.
  • Reliability– The reliability of the interface is a fraction of 255. If the value is 255/255, it means that the reliability is 100%. The reliability is calculated as an exponential average over five minutes.
  • Txload, Rxload– This is transmitted and receive load on the interface as a fraction of 255 but 255/255 is completely saturated. The value is calculated as an exponential average over five minutes.
  • MTU – The MTU (Maximum Transmission Unit) is not used in the metric calculation formula. The default value of the Maximum Transmission Unit is 1500 Bytes. The Maximum Transmission Unit is carried in the EIGRP Update message to be used when tie occurred.
Show Interfaces