[mrtg] Re: Cisco 7500 POSIP reporting

[Alex van den Bogaerdt]

> When I first startup MRTG polling on the POS , it initially reports the
> correct inbound bit rate, but on the next interval, reports errantly
> low values.  For example, the first interval reports a correct inbound
> rate of 107 Mbits/sec, then the next interval will report the current
> inbound rate between 2-3 Mbits/sec.  Over the next few intervals, the
> rate increases to around 15-20 Mbits, but it is still way off the actual
> 107+ Mbit rate.
> 
> However, the outbound rate is reported correctly the whole time, which
> is approx 40 Mbits/Sec.
> 

You monitor every five minutes, this is 300 seconds.
A rate of 107 Mb/s is 107 * 1000 * 1000 / 8 MB/s is 13375000 B/s.
300 seconds @ 107Mb/s is 4012500000 B/s.
                 2^32 is 4294967296 ... oops, darn close.
The equivalent rate for 2^32 is 114.53 Mb/s.
Most likely you are having problems with this.

For 115Mb/s (so: just over the 2^32 problem):
Time    counter     
T+000         0
T+300   115 * 1000 * 1000 / 8 * 300 = 4312500000
T+600   2*4312500000                = 8625000000

However, the counter is 32 bit so it becomes:

Time    counter     
T+000         0
T+300   115 * 1000 * 1000 / 8 * 300 = 4312500000 mod 2^32 = 17532704
T+600   2*4312500000                = 8625000000 mod 2^32 = 35065408

When these values are stored, they represent a rate of about
17532704 / 300 == 58442 B/s == 58.4 kB/s

What most likely happens is that the graph is just 114.53 Mb/s short ...

There are a few options:

1) Monitor more often (just call the mrtg script every two minutes) so that
   the counter won't increase with values over 2^32.
2) Switch to rrdtool

HTH
Alex

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