Lets play with gns3 to implement a small playground for our cisco study. First thing will be preparing a router to use. Choose a router whose ios is loaded to gns3 before, drop it to center plane of gns3 and with right clicking, choose “configure” option. At “slots” section, we may add interfaces to our naked router. For the moment, just add a serial interface, NM-4T.
To see the result, we may start the router and then open a console using right click menu of router. Since our router is blank, choose ‘no’ to routers ‘initial configuration dialog offer’, go to previledge menu with ‘enable’ command, and see the newly added interface card with ‘show ip interface brief’ command,
Initially, emulating a router in gns3 will consume much of systems cpu clock. To set the correct calculation speed, something called “idlepc” value should be set accordingly to save cpu cycles. This calculation should be the first step before using a newly added ios. Since our ios is new, and we do not want to drain all drops of our cpu cycle, we select “Idle PC” option, and let gns3 to suggest an idlepc value. Having option to choose doesn’t mean to just pick a random value, so unless you have a good reason, it’s better to select the gns3 suggested one (which is marked with asteriks).
Resulting idlepc value may be seen from “Edit” > “IOS images and hypervisors” menu option
Having an idlepc configured router, we may now proceed to implement the simple topology figured below,