I have a few tips for those who want to give this a shot. First, Dynamips can put a hurt on your host CPU resources. This is because the emulator essentially runs full-steam for every emulated router, no matter what the processing requirements for that router may be at the moment. Even an idle router is using 100 percent at the emulation level. There's a facility to combat this, called Idle PC, which seeks to determine the router's functional level when idle. When this is properly configured, the CPU utilization drops dramatically. Second, it pays to read the forum threads carefully when peeling apart your ASA images. There are a few scripts that can greatly ease the initial setup.
I did run into a few bugs here and there, involving the occasional inability to delete links if one of the routers on that link was deleted prior to the link itself. Nevertheless, generally speaking, GNS3 is fairly tight, considering the work it can do. Finally, there are a few operational limitations. For instance, GNS3 doesn't support Cisco 2800- and 3800-series routers, or ASA version 8.2 or later as of now, but that might change in the future.
As it stands, I was able to use GNS3 to build and thoroughly test a large, complex network topology using real addressing, and then save off my configurations. When Cisco finally does find a few ASA 5510s for this project, I'll be able to drop those configs on them and have the network up and running extremely quickly. Naturally, GNS3 is also an excellent way to test proposed network configuration changes, and it's an easy way to train for Cisco certification exams. If you have the required IOS or Juniper images, it's worth the time.
This story, "Emulating Cisco networks for fun and profit," was originally published at InfoWorld.com. Follow the latest developments in networking, read InfoWorld Test Center product reviews, and dive into Paul Venezia's The Deep End blog at InfoWorld.com.