[mrtg] Re: Cisco Switch: Interfaces don't add up right

Eric Brander eric_mailing_list at rednarb.com
Thu Apr 7 15:55:32 MEST 2005

Steve WOODIN wrote:

>>>>Shane Presley <shane.presley at gmail.com> 4/7/2005 9:46:17 AM >>>
>When we fire off our backups at midnight, we see lots of ports max out
>(95% utilization or about 11MB/s).  They all are sending data to our
>uplink port (media server).  But what's odd is the gigabit uplink port
>only shows 10% utilization (11MB/s).

>Umm, because it's a switch and not a hub?  Remember the way switching
>technology operates.  It tries to preserve the bandwidth and keep each
>connection "private" and not share it like a hub would. Therefore you do
>not "add up" the total data going in, because the switch, if at all
>possible, is going to try to preserve the speed for each connection. 
>So in your case:
>Server 1 - Media Server = 11Mbps
>Server 2 - Media Server = 11Mbps

He said 11 MB/s  MegaBytes per Second. Your example is in bits and 
doesn't quite apply.  I ~think~ the answer to his question is in the 
32-bit counter wrap problem. I forget the exact number but at somewhere 
north of 109 megabits per second, the 32-bit counter will wrap twice 
within a 5-minute period, giving inacurate results.

Try :::::2 at the end of your target string as per the reference.html 
and see if your gig interfaces support 64-bit counters.


Eric Brander

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