[mrtg] Re: Cisco PortChannels

[Alex van den Bogaerdt]

Grendel wrote:

> Since then I did the math and figured out that MRTG using SNMPv1 and
> the default 5 minute interval can graph a max of 114 Mbps. This would
> explain the region of the graph that hovered at a measly 300 Kbps. But
> it doesn't explain the ceiling it hit at almost 100 Mbps on the other
> side of that region.

You also need to know that MRTG will discard negative intervals. It
will use the previous value instead. When you have traffic approaching
114kbps, you get frequent counter wraps.  Each time the counter wraps,
it will produce a negative delta which is discarded.

Do the math again. Start at counter=0, delta is 99 Mbps.

> 	Is there someone with experience monitoring or using Cisco port
> channels who could explain what I'm seeing? When you tie two ports
> together in a port channel on a Cisco and a 've' interface on a Foundry
> BigIron at the other end, do you in fact get a load-balanced, cumulative
> bandwidth of 200 Mbps, or just a load balanced 100 Mbps?

It is my understanding that you get 200 Mbps but I never saw this kind
of traffic on our switches. I'm not so sure that you get a load balanced
link by the way (perhaps I should tweak some settings?).

cheers,
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