GNS3 topology.net file analyzing for Macintosh user

Basic setup for GNS3 has already done in the previous article.
In this article, will take a look inside the .net file.

A .net file is a topology configuration file for GNS3 so that you should know how to tune them, otherwise you need limitless CPU power. You don't want to burn your wrist on palm rest, hah?

To open the .net file, you might want to use your favorite text editor. It supposed to open as a simple text, not rich text.

A .net file has four sections.
　1. PROCESS
　2. MODEL
　3. RESPECTIVE ROUTER
　4. COORDINATE

You may need above three and coordinate is not that important for you to setup GNS3.

1. PROCESS

The process is a most left sided part of the .net configuration. This section has a Router model and respective router inside. The process is what it means is as it is, you make five processes, your Macintosh will run five Dynamips processes.

The .net file above has [127.0.0.1:7200], which means the port 7200 is mapped into localhost, otherwise, you can create multiple processes with changing those ports as needed(like 7201, 7202...).

Also, you need to change udp port which is 10001 above. You should add 100 for each udp port like 10101, 10201... These ports must be unique ports.

If you bring several routers like 20 or 30 into your topology, GNS3 sometimes automatically determine to create several processes. Anyway, in a single process, it is better to have at most five routers inside.

2. MODEL(router)

At the model section, there is a path of IOS and booting method of respective routers. You won't need to change those parameters but to add idlemax parameters.

You see the idlemax=100 is inserted in the model section, below.

    [[3640]]
        image = /Users/shun/Documents/IOS/c3640.image
        sparsemem = True
        ghostios = True
        chassis = 3640
        idlemax = 100

After inserting idlemax = 100, save an overwrite the file.

I did a simple test if idlemax is effective or not.
I made two patterns of topology.net file, one has "idlemax" and one has not. Boot both respectively and check the CPU usage after doing idlePC calculation.

No idlemax.

idlemax inside.

As you see, without idlemax, the cpu usage is around 25% but 8% with it.
This test only simulate four routers so, it does not have an extreme difference with them. But think if  you run 20 or more routers, which makes lots difference as you can guess easily.

3. RESPECTIVE ROUTERS

Respective router section has nothing to tune, but you should remember one thing.
With using terminal soft, GNS3 says that is the console connection but it is just a virtual console connection and this mean just a telnet connection.

Take a look inside the red box above, it says console = 2104. This means you can connect this router with telnet 127.0.0.1:2104. If you know this information, you can setup your topology to make every router to use each port number every time. Otherwise your terminal soft bookmark will be stable and just click the bookmark and you can connect the wishing router without any additional operation.

You might want to practice same topology or workbook repeatedly, bookmark will not bother you to search one objective router out of 30 or more router in the topology and click to connect it, imagine how annoying you l it is.

I always use 2101 for R1, 2102 for R2. I always map most right two number to map the respective router number. Sometimes you want to download the workbook topology from somewhere, it has some other rules which annoys you to connect. Otherwise your stress could be lower, if you know those settings.

4. CREATE MULTIPLE PROCESSES

As I mentioned before, this test only simulate four routers, so there will not be effective to separate each router to respective process.
Anyway, separate them and let's see what happens.

Result above.
You see four Dynamips processes and each process use around 3% of CPU power.
It is just for four routers but if you run 30 or more, you can easily guess it will work effectively.