Load-Interval

The 'show interface' is probably one of the most commonly used troubleshooting commands, but as we previously discovered, some things aren't as straightforward as you would expect or hope.  The load-interval is a perfect example of this.  Whether you leave the default value of 5 minutes, or change it to 30 seconds, it's not as simple as the average load over the configured time.

If you put a traffic generator on an interface, blast 1 gbps of traffic down a Gig-E pipe, you'd expect the load-interval to reflect the actual load a few seconds after the interval, right?  It's not quit that simple, and could take many times the configured value to reflect an accurate load.

The formula for the load-interval is actually a decayed/weighted average.  The formula is as follows:

Rate = ((Previous Average - Current Average) X EXP(-[interface update] / [Configured Load Interval])) + Current Average

Let's start with an example.

Gig-E Interface:            Previously Average (0 mbps)
                            Current Average (1000 mbps)
Configured Load Interval:   30 Seconds
Interface Update:           5 Seconds

The interface update is how frequently IOS updates the value in the 'show interface', which seems to be 5 seconds on many platforms.

We'll start by calculating the decay value (the EXP part of the equation) first:
    EXP(-5/30) = 0.846481725

The formula now becomes:
    Rate = ((Previous Average - Current Average) X 0.846481725) + Current Average

    153,518,275 =  (0 - 1,000,000,000) x 0.846481725 + 1,000,000,000

I opted to put this in to Excel and fill out 90 seconds worth of values, to see how long the ramp up time is.

Seconds   Load Interval
   5       153,518,275
  10       283,468,689
  15       393,469,340
  20       486,582,881
  25       565,401,791
  30       632,120,559
  35       688,596,776
  40       736,402,862
  45       776,869,840
  50       811,124,397
  55       840,120,254
  60       864,664,717
  65       885,441,156
  70       903,028,032
  75       917,915,001
  80       930,516,549
  85       941,183,528
  90       950,212,932

As you can see, it can take over 90 seconds to report a value close to the actual utilization, even though we've configured a 30 second load-interval on the interface.  If you want a more accurate representation of interface utilization, you should look to your SNMP management tools.