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In using Dynamips while now while I study for my CCNP and discovered that even though with a good idle_pc value, the CPU utilisation spikes after the con session times-out. I initially noticed that once the router boots and you’re prompted ‘Press RETURN to begin!’ and the interfaces normalise, the CPU stays pretty high until you press RETURN to begin the console session. Same goes for when the console session times out.

Running 6 2600s at ~30% processor usage and having one session time out brings the CPU usage up to 90% until I press enter again to clear the ‘you’ve timed-out’ message. If several time out, the overall load goes up pretty quick, 7+ at times.

The key is to just set the exec-timeout to 0 -

Router(config)#line con 0
Router(config-line)#exec-time 0 0

With avoiding the time-outs, I should be able to load twice the number of routers now.