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.
I've been working with cisco equipment for 3 years and because of your blog i found out about the load-interval command. Excellent!
ReplyDeletenot understand can u pls explain with more details and example
ReplyDeletehi James Ventre can u explain you calculation little bit more detail buddy i couldn't get how 0.846481725 by dividing 5/30 i get some other valuse then how other calcultion move pls replay
ReplyDeleteYou don't get 0.846481725 by way of 5/30. It's "EXP(-5/30)", EXP is explained here: http://www.techonthenet.com/excel/formulas/exp.php
ReplyDelete