Install and Configure GNS3
First, You should install gns3:
sudo emerge --autounmask-write gns3
sudo dispatch-conf
sudo emerge gns3
Then you need to prepare for ios images, You can download them from:
ftp://ftp.unikon-ua.net/pub/Cisco/IOS/
http://115.com/file/clsclmjb
Then you should configure them:
Edit>IOS Images and Hypervisors
And calculate ‘Idle PC’。
You may want to get rid of warning of baseconfig.txt, refer to:
http://forum.gns3.net/post12717.html
If you are using KDE, you may use konsole’s preconfigured terminal commands.
Edit>Preferences>General>Terminal Settings
You need to install and configure virtualbox at Edit>Preferences>VirtualBox:
- modify - Path to vboxwrapperto:- /usr/libexec/gns3/vboxwrapper.py
- emerge virtualbox with sdk and python USE flags 
- install vboxapi: - cd /usr/lib/virtualbox/sdk/installer/ export VBOX_INSTALL_PATH=/usr/lib/virtualbox/ python vboxapisetup.py install
- install xdotool(It’s quite an interesting tool!) - sudo emerge xdotool
- add virtual system:Edit>Preferences>VirtualBox>VirtualBox Guest
You need to install qemu at Edit>Preferences>Qemu:
- modify - Path to qemu:- qemu-system-x86_64
- modify path to qemuwrapper to: - /usr/libexec/gns3/qemuwrapper.py
- kernel should support kvm-intel: - ⮀ ~ ⮀ lsmod|grep kvm kvm_intel 117933 0 kvm 225670 1 kvm_intel
- and add you user to: - gpasswd -a username kvm 
- add virtual system, which can be - vdi
- If you want to use PIX firewall, download pemu from Sourceforge and cp them to the directory which has - qemuwrapper.py. You can reffer to: http://www.gns3.net/documentation/gns3/pix-firewall-emulation/
- if you re-compile kernel, don’t forget to re-compile kernel modules for virtualbox and modprobe them 
More configuration
More configuration and downloads(I tried and failed, do not know what is actually is and how to use it):
A Simple Test
- Calc idle PC, very important!!
configure the router with the help of OSPF configure(中文)
A better choice to learn without these configuration, try GNS3 WorkBench