HCC Cisco Academy: IGRP Metric Calculation

The metric used by IGRP is 24 bits long (1 to 16,777,215).  As with RIP, the lower the value the better the route.  This metric is determined from the following 4 quantities:

Bandwidth This is the speed of the link.  Unless the bandwidth command is given in the interface configuration, the bandwidth defaults to T1 speed (1.544 Mbps).  This should be set on serial ports even if you don't set the clock rate.  The value used in the metric calculation is the smallest bandwith of any link in the path of the route.  Since the unit for bandwith is millions of bits per second, and the other factors used have units of milliseconds per bit, IGRP will divide 10,000,000 by the bandwith and use the result (now with the correct units) in the metric caclulation.
Delay This is a measure of the latency inherent in the transmisison facilities used on the route, in milliseconds per bit.  The delay value used is the sum of all delays for each link along the path of the route.  The default delay of any link is determined by the technology used for that link and the bandwidth (show interfaces will report the delay for each port).  The delay can be set to reflect the actual latency of a link (any value from 1 to 16,777,215).  Note that a shorter T1 link has a lower delay than a longer T1 link, and a 10 Mbps ethernet link has 10 times more average delay than a 100 Mbps one.  Use the delay interface configuration command to change the delay.
Load The load is a measure of the current utilization of the heaviest used link in a route.  It can vary from 1 to 255.  The higher the load on a route, the longer it will take to send a packet along that route.  However, load tends to vary rapidly.  If you can anticipate that a link will have a high average load (utilization), you may wish to configure this factor.
Reliability This is a measure of the percent of packets with errors to total number of packets on a link.  The value used in the metric calculation varies from 1 to 255, and can be controlled somewhat with the clear counters command.  The reliability of the worst link along a route is the reliability of the route.

In addition to these factors, hop count and MTU are reported in IGRP route updates.  (The value for MTU is the smallest MTU of any link along the route.)  These factors are not used in the route calculation.

Beside these factors a set of five multipliers are used named K1 through K5.  These can be set with the metric weights router configuration command.  The default values are:  K1=1, K2=0, K3=1, K4=0, K5=0.

The formula used is:

bandwidth = 10000000 ÷ actual-bandwidth

Metric = (K1 × bandwidth) + (K2 × bandwidth) ÷ (256 - load) + (K3 × delay)

If K5 is not equal to zero, then the above Metric is modified:

Metric = Metric × ( K5 ÷ [ reliability + K4 ] )

Note EIGRP uses the same metric, but scales it by 256.

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