Beyond a pure percentage, you might want to account for the time it takes you stay below a certain threshold. If you want to target a certain link to keep your 95th percentile peaks below 70%, then first get an understanding of your traffic growth and try to project when you will reach that number. You have to decide whether you care about the occasional peak, or the consistent peak, or somewhere in between, like weekday vs weekends, etc. Now you know how much lead time you will have.

Then consider how long it will take you to upgrade that link. If it's a matter of adding a couple of crossconnects, then you might just need a week. If you have to ship and install optics, modules, a card, then add another week. If you have to get a sales order signed by senior management, add another week. If you have to put it through legal and finance, add a month. (kidding) If you are doing your annual re-negotiation, well...good luck.

It's always good to ask your circuit vendors what the lead times are, then double it and add 5.

And sometimes, if you need a low latency connection, traffic utilization levels might not even be something you look at.

Louie

Peering Coordinator at a start-up ISP

On Fri, Aug 14, 2020 at 4:13 PM Radu-Adrian Feurdean <nanog@radu-adrian.feurdean.net> wrote:

On Wed, Aug 12, 2020, at 09:31, Hank Nussbacher wrote:
> At what point do commercial ISPs upgrade links in their backbone as
> well as peering and transit links that are congested?  At 80% capacity?
>  90%?  95%? 

Some reflections about link capacity:
At 90% and over, you should panic.
Between 80% and 90% you should be (very) scared.
Between 70% and 80% you should be worried.
Between 60% and 70% you should  seriously consider speeding up the upgrades that you effectively started at 50%, and started planning since 40%.

Of course, that differs from one ISP to another. Some only upgrade after several months with at least 4 hours a day, every day (or almost) at over 95%. Others deploy 10x expected capacity, and upgrade well before 40%.