[mrtg] Re: Graphing Gigabit Interfaces

greg mrtgmail at gschmidt.net
Tue May 23 04:09:23 MEST 2006

Dan Mahoney, System Admin wrote:
> Hey all, I'm trying to graph some gigabit interfaces on my cisco 6506 
> 8-port gigabit blade and the graphs are maxing out at about 97 Megs.
> I noticed that I needed to use SNMPv2, but this hasn't seemed to alleviate 
> the problem, as things still get set thusly:
> ### Interface 10 >> Descr: 'GigabitEthernet3/8' | Name: 'Gi3/8' | Ip: 
> '' | Eth: '' ###
> Target[]: 10:h0stm3 at
> SetEnv[]: MRTG_INT_IP="" 
> MRTG_INT_DESCR="GigabitEthernet3/8"
> MaxBytes[]: 125000000
> ^^^ The above looks right, as in it's 10X any fast ethernet interface 
> (which mrtg gives FIVE zeroes) -- but it's still 125 million bits, which 
> is 125 megs, right?
> Title[]: Traffic Analysis for 10 -- ads-bsh-cr01
> PageTop[]: <H1>Traffic Analysis for 10 -- ads-bsh-cr01</H1>
>   <TABLE>
>     <TR><TD>System:</TD>     <TD>ads-bsh-cr01 in </TD></TR>
>     <TR><TD>Maintainer:</TD> <TD></TD></TR>
>     <TR><TD>Description:</TD><TD>GigabitEthernet3/8 Uplink to Arbinet 
> (AS18775) via han-bsh-cs05-g0-2 </TD></TR>
>     <TR><TD>ifType:</TD>     <TD>ethernetCsmacd (6)</TD></TR>
>     <TR><TD>ifName:</TD>     <TD>Gi3/8</TD></TR>
>     <TR><TD>Max Speed:</TD>  <TD>125.0 MBytes/s</TD></TR>
> ^^^ And no matter what I do, this doesn't get fixed either.

125.0MBytes = 1000Mbits = GigaBIT Ethernet.

125 * 8 = 1000

You might want to use the "bits" option in cfgmaker if you want to see 
bits per second in your graphs instead of bytes.  It will then multiply 
the bytes by 8 to get bits.  The issue is that SNMP records and reports 
the amount of traffic through an interface in octets, which are 
basically the same as bytes, or 8-bit words.  MRTG needs to multiply 
this number by 8 to get the number of bits, which is how most folks 
think of it.

Good call on using SNMPv2 to get the 64 bit counters that won't roll 
over inside of five minutes on a GigE interface.

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