Hello,
the 70% average is a good rule of thumb. What is meant by 70% is worth thinking about. Do you average busy hours or office hours or daily? The result can be quite different. I would recommend to take busy hours. The average can also be misleading. Assume - for simplicity - a 2 minute overload situation with lots of dropped packets and the 3 minutes idle. It will give you less than 50% usage in a 5 minute interval.
What you should do additionally to monitoring average load is to monitor the dropped packets on the link because of congestion. Once your router starts dropping a considerable amount of packets it would be about time to upgrade. In fact when you have the data at hand you can forsee, when the upgrade will be needed by looking at the extrapolation of the usage data.
You could also apply QoS to the link and make sure the critical applications get their required bandwidth even in an overload situation. Be aware that in this case you have to drop other less important traffic.
In any case the application response times and user input might also give a hint on when you would need to upgrade.
Hope this helps! Please rate all posts.
regards, Martin
