Link capacity upgrade threshold

Patrick W. Gilmore patrick at
Sun Aug 30 12:03:35 CDT 2009

On Aug 30, 2009, at 1:23 AM, Mikael Abrahamsson wrote:
> 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 you.
> 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.

I've heard this said many times.  I've also seen 'sho int' say  
950,000,000 bits/sec and not see packets get dropped.  I was under the  
impression "show int" showed -every- byte leaving the interface.  I  
could make an argument that IFG would not be included, but things like  
ethernet headers better be.

Does this change between IOS revisions, or hardware, or is it old  
info, or ... what?


P.S. I agree that without perfect conditions (e.g. using an Ixia to  
test link speeds), you should upgrade WAAAAAY before 90-something  
percent.  microbursts are real, and buffer space is small these days.   
I'm just asking what the counters -actually- show.

> 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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