[mrtg] cisco switches, etherchannel, and artificially low graph values?

Kettler, Matt Matt.Kettler at edocorp.com
Fri Mar 21 15:47:46 CET 2008

I've got an odd problem with my interface graphs never reaching their
proper peak.. They all seem to be scaled down inappropriately.


For starters, I've pointed cfgmaker at a variety of ordinary Cisco
switches.  I'll be using examples from a Cisco 4948 running
12.2(20r)EW1, but I've seen the same affects on 3750's 12.2(25r)SE1.


I've got MRTG running in daemon mode at 5 minute intervals.


When I run cfgmaker, I use this basic form:


cfgmaker --global 'WorkDir: /var/mrtg/work' --global
'Options[_]:bits,growright' --global 'WithPeak[_]: wmy' --no-down
--show-op-down --output  switchname.cfg community at switchname


This generates me a nice little switchname.cfg file, so far so good.


It properly adds the global Options[_]: bits, growright, which makes
MRTG graph in bits, not bytes, which I prefer, etc..


Cfgmaker generates an entry for each port (ie: gi1/3, gi1/4), and one
for the combined port (ie: po7). However, MRTG seems to handle the
combined ports strangely.


This particular example is a 2gbit/sec LACP port made of 2 gig-e
interfaces. cfgmaker understands this port and generated:


MaxBytes[switch_96]: 250000000


And pagetop contains:

                                        <td>Max Speed:</td>

                                        <td>2000.0 Mbits/s</td>



Which is all as it should be. When MRTG runs, it generates the graphs
which claim to be in bits, but the values never reach as high as they


When I log into the switch and use sho int I get:


5 minute output rate 208729000 bits/sec, 17265 packets/sec



That's 208mbit/sec, or perhaps 199 mbit/sec if you count by 1024's. This
is a port to my backup server, and it sustains rates around
300-500mbit/sec pretty readily.


However, MRTG is reporting this on the daily graph:

            Max      Average             Current

Out       143.0 Mb/s (0.9%)         23.9 Mb/s (0.1%)           75.1 Mb/s


Earlier I had the switch reporting 463mbit/sec for an extended period of
time, and I've not gotten a report out of the switch below 200mbit/sec
in repeated queries over the past hour minutes or so. However, even the
24hr max doesn't line up with my current measurements.....


Both are operating on 5 minute averages, and the load is fairly
sustained, so why the low numbers in MRTG?



Also, in the same graph update MRTG reports:

                                    Max      Average             Current

Gi1/3                Out       106.8 Mb/s (1.3%)         18.9 Mb/s
(0.2%)           96.3 Mb/s (1.2%)

Gi1/4                Out       165.6 Mb/s (2.1%)         16.7 Mb/s
(0.2%)           46.7 Mb/s (0.6%) 

Po7                  Out       143.0 Mb/s (0.9%)         24.3 Mb/s
(0.2%)           46.2 Mb/s (0.3%)


How does an etherchannel have a lower current output rate than either of
its children?


Is there a known issue on cisco switches and their reporting of
etherchannel interfaces over SNMP?


In general I seem to be having anomalous numbers on all my switch ports,
but they're most noticeable on etherchannels

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.oetiker.ch/pipermail/mrtg/attachments/20080321/ce96b19c/attachment-0001.html 

More information about the mrtg mailing list