People who are not aware of GNS3 , just to give a brief about GNS3.
GNS3 is a graphical network simulator that allows simulation of complex networks.
GNS3 is an excellent complementary tool to real labs for network engineers, administrators and people wanting to study for certifications such as Cisco CCNA, CCNP, CCIP and CCIE as well as Juniper JNCIA, JNCIS and JNCIE.
It can be used on multiple platforms and its an opensource so we can download and use it for free.
After you download and install GNS3, we also need to have real Cisco IOS router images. For all my lab purposes I use image for Cisco 3725 router. You can download it from Cisco , but you need to be a have account with Cisco or for Lab purpose if you can get an image from some where.
One more very important thing we have certain limitations using GNS3 that we can't emulate certain things we use in VoIP like pots ports, E1, T1 and Cisco Unity and more.
But the best benefit I see with GNS3 is I can connect it my computer, and integrate with my Call Manager. That makes it really wonderful when you are doing lab for Voice certifications
After you download GNS3 just follow two steps to get started:
Go to Edit -- Preferences--Dynamips--Test Dynamips (It shows test successful)
Go to Edit--IOS images and hypervisors . Select correct router and related image.I am using a 3725 image for 3700 platform. Try to make a default RAM 128Mb for your voice routers.
Image Name: c3725-adventerprisek9-mz.124-15_T13_unZ.BIN