What can I use (using MRTG) to see how much bandwidth is utilized at the serial port of the router?
The alternative is to use an end-to-end performance measurement tool to sample the true available bandwidth at various times to see how the pipe is actually being filled. My recommendation would be appareNet of course but I'm biased. Combined with MRTG or other point source readings, this is your best source of information. If you can access counters on your switches then you are all set.
The benefit of the end-to-end approach is to let you see if the router, or some other point, is acting as the bandwidth bottleneck. If 50 workstations are already saturating the link, then the MRTG measurements associated with 75 workstations are not linearly related to the measurements for 50. This makes it difficult to determine how your provisioning should scale. If there is no apparent saturation at any point, then you can fairly safely assume that your network utilization is approximately linearly dependent on the number of workstation.
Since there you know the topology of your cascaded system, measuring end-to-end along different switch paths can help you determine if/where saturation is taking place in the switch network.
This was first published in April 2003