Link capacity upgrade threshold

Mikael Abrahamsson swmike at
Sun Aug 30 00:23:57 CDT 2009

On Sun, 30 Aug 2009, William Herrin wrote:

> If your 95th percentile utilization is at 80% capacity, it's time to 
> start planning the upgrade. If your 95th percentile utilization is at 
> 95% it's time to finish the upgrade.

I now see why people at the IETF spoke in a way that "core network 
congestion" was something natural.

If your MRTG graph is showing 95% load in 5 minute average, you're most 
likely congesting/buffering at some time during that 5 minute interval. If 
this is acceptable or not in your network (it's not in mine) that's up to 

Also, a gig link on a Cisco will do approx 93-94% of imix of a gig in the 
values presented via SNMP (around 930-940 megabit/s as seen in "show int") 
before it's full, because of IFG, ethernet header overhead etc.

So personally, I consider a gig link "in desperate need of upgrade" when 
it's showing around 850-880 megs of traffic in mrtg.

Mikael Abrahamsson    email: swmike at

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