Re: [cricket-users] Possible feature?

From: Alan Nichols (Alan.Nichols@EBay.Sun.COM)
Date: Mon Jul 12 1999 - 19:06:50 PDT


From: Alan Nichols <Alan.Nichols@EBay.Sun.COM>

Ivan:

>From the problem you describe, I take it you are using Cisco switches
and from time to time go into the switch and do a 'clear counters'
command on the switch. Doing this resets the counters you see in the
CLI and visible through SNMP to 0. This will cause RRD to see a 'spike'
in traffic because it can not tell the difference between the counter
being reset and the counter rolling over normally.

There are two ways to deal with this: set a max value, or don't clear
counters on your switch. A third way would be to complain loudly and
long to Cisco for creating products that are not compliant with the
SNMP standard.

Yes, you read that right - On this point, Cisco switches do *not* comply
with the SNMP standards. (Verifying this will take a bit of searching,
but the key is in the definition of the 'Counter' datatype in the SMI
RFC's, and in the definitions of 'ifInOctets' and 'ifOutOctets' in RFC1573).

Specifically, by the SNMP standard, a counter should *never* get reset
to 0, unless you have some way of detecting that reset. Cisco switches
will reset their counters to zero and won't tell you that anything happened.

(Note: I submitted a complaint to Cisco about this and got a kinda snide
'we like the way it works and will not change it' answer. And they
rewrote the IF-MIB so that they can get away with it in the future by
implementing a 'ifCounterDiscontinuityTime' variable).

My recommendation: do not clear the counters on your switch.

Alan

>From: "Berg, Ivan" <iberg@montana.edu>
>To: "'cricket-users@onelist.com'" <cricket-users@onelist.com>
>Mailing-List: list cricket-users@onelist.com; contact
cricket-users-owner@onelist.com
>Delivered-To: mailing list cricket-users@onelist.com
>List-Unsubscribe: <mailto:cricket-users-unsubscribe@ONElist.com>
>Mime-Version: 1.0
>Subject: [cricket-users] Possible feature?
>
>From: "Berg, Ivan" <iberg@montana.edu>
>
>First of all, thanks Jeff for the quick response time on 0.68. It is very
>handy being
>able to talk to developers first hand.
>
>Hey, do you think this could be added in a future version of cricket?
>
>The problem is when a counter get's reset to zero. This produces a large
>spike in the
>graph which throws off the range and average's for some time.
>
>It seems that was is stored in the data file is differences in data and this
>is divided
>by time to give us something like data/sec. So it works my doing
>(current data - last data)/ time. Now this value is positive when the
>counter is
>incrementing and only negative when the counter gets reset to zero. Would
>it be possible
>to add small routine to check if there is a negative difference and account
>for it.
>
>Here a couple ideas to handle the negative and incorrect values.
>1. Throw away the data point, i.e don't store it in the data.
>2. Set point to 0 to avoid spikes, it would still slightly affect average
>but not as
> much as large spike.
>3. If detect negative value, then average the last two data points and
>store that
> as the current data point. That would be the most accurate of the three
>and
> would not throw off the scale at all.
>
>Thanks
>
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-------------------------------------------------------------------------
Email: Alan.Nichols@Sun.COM Phone:(408)276-1306
Pager: Alan.Nichols@pager.ebay.sun.com or (650)940-0687
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