[mrtg] Re: strange values with sub-interfaces

[Félix Izquierdo]

Olivier Gontier wrote:
> 
> Hello,
> 
> I use Mrtg 2.8.8 to monitor serial interfaces on a cisco 7507;
> when I look the results, there is a big difference
> between Serial6/0/0 and a sub-interface Serial6/0/0.6 .
> 
> For example:
> 
> Serial 6/0/0
>  Current In : 434 kb/s
> 
> Serial 6/0/0.6
>  current In : 1018 kb/s
> 
> It's not logical because In Serial values should
> be the sum of In sub-interfaces values .....
> 

No if you are working with compression. Snmp counters for FR subinterfaces are
layer 3 counters. Physical interfaces counters are layer 2 counters. The normal
situation is a little difference between physical values ( a bit more high ) and
the summ of all the subinterfaces, because LMI and layer 2 overhead, but if you
are using payload compression in some DLCI, then you get the inverse effect
because physical-layer2 values are compressed and subinterface-layer3 are
uncompressed.

If you want to get true layer 2 per-pvc values, you must use the specific
per-pvc counters in FR MIB instead of subinterface counters in the standard
interface MIB. Getting both values, you'll have the compression ratio.

Cheers.

Félix
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