Dynamips idle-PC tuning: stop a router using a whole CPU core
Dynamips copies classic Cisco hardware by reading IOS instructions one by one. Without a correct idle-PC value (a setting that tells it when the router is idle), it uses a whole CPU core even when nothing is happening in the lab. netplex. calibrates this automatically the first time a Dynamips router boots, then lets you pick from the real measured candidates.
Select the router, open Config → Hardware. Once calibrated, Idle-PC becomes a dropdown of real candidate values, each with the CPU% netplex. measured for it - lower is better.
Pick a candidate from the dropdown - it saves immediately and takes effect on the router's next restart (Dynamips has no live-apply for this).
Calibration and the saved value are cached permanently per image, not per lab - once tuned, every future router using this same IOS image starts light. Only QEMU-based images (vIOS, CSR) never need this; the idle-PC problem is specific to Dynamips's instruction-level emulation.