How to write long iterative configs

By | May 12, 2015

Sometimes there is a problem: you need to write a really repetitive config with hundreds of nodes. Clear example would be dial peers in Cisco routers. It is immensely hard and dull to do this by hand or copy+paste. Here’s a simple python script to do it for you.

Iterative config generation

I prefer to solve such problems with some scripting, and my go-to language for that is Python. Also, this script is a clear example of data-code separation. I store the data used to generate the config in a separate file ( from the generator code itself ( Let’s look at them closely.

Data used to generate config

First, we need data to generate a config from. In my case, the data is divided in two types:

  1. Config template – this is the part which the script will iterate
  2. Data lines – a table of the parts of config that need be changed in each iteration, like IP addresses and description lines

The config template is basically a multi-line string, which stores the config almost just like you’ll put it into your device’s console. The only difference is that at each place where a substitution from the data table is expected, we place a special tag: {tagname}.

Here’s an example of dial-peer configuration for CUBE used in my script:

Here, the

variable stores the template itself. Nothing fancy, except for the tags (sequence, description, pattern, ip) mentioned above

Moreover, I used a separate Python dictionary to store static variables that get placed into config. It is a little overkill for my task at hand, but it seems to be reasonable to write it in that way for any future use. Here’s how it looks like:

Here one can add any number of static tags. As I show it later, the {ip} tag in the template is replaced by the data stored here under [ip]. Also, any static stored data here will take precedence over data lines for the same tag. Aside from that, my config template stores an offset for the counter (which is filled into the sequence tag), as my task was to generate a config to be appended to the similar config already in place. The use of a counter here is obvious: there is no point in storing this number with the rest of the data used to generate the config. Additionally, not used in my example, but nevertheless present in the script (for future use) are two variables:

Their meaning should be evident from variables’ names: one prepends the iterative part, and the later is appended to the end.

The data lines file is a simple CSV-table. The first line, being the table header, stores the {tags} for respective columns. In my case it looks like this:

In the form of table that would be:

97520…….$IN, Indianapolis
97513…….$IN, Smalville
96412…….$IN, Regentsville

Nothing fancy here.

The script used to generate config

The script has two main procedures:

  1. main() – as the name suggests, this is the main part which reads the data file, writes the resulting config and iteratively calls the
  2. configblock(dataline) procedure takes the iterative part of the template (stored in CONFIG_TEMPLATE_ITERATIVE) and formats it (in my case – replaces {tags} with respective fields from the dictionary.

The configblock(dataline) procedure is very simple:

That’s all. Here the procedure just returns the result of

method of a string variable. This could even be done in-line at the main procedure, but I sacrificed a little efficiency for a little more readability. And here’s how the main() procedure goes (I removed the lines concerned with the little user interface the script has, as they are not crucial for the script’s work):

First, the script reads the data file. The file is read into a list of dictionaries, which allows for two important things:

  1. It can be easily iterated through
  2. At each iteration we can find the iteration[‘tagname’] (see further)

Next, the sequence number variable (used later as a counter) is filled with an offset and the config variable (used to store script’s results) is pre-populated with the CONFIG_TEMPLATE_ADD_BEFORE data.

Now, the main part. Here the iterative generation is performed:

  1. take each line from the DataFileReader – i.e. each line of the table in order
  2. append sequence number from the counter
  3. append any static variables from CONFIG_TEMPLATE_STATIC, replacing those with the same tags
  4. increment the sequence counter
  5. format new portion of iterative part of config (via configblock(line) ) and append it to the config variable

Next, the tailing part of the config is added (in my case it is an empty string, so just a new line is added).

Lastly, the config is written to disk into the config.conf file. End of program.

The full code of this nice iterative config generator

Below I provide the code itself and some links for those who might find this simple script usable.

The code

The links

The script is on GitHub in my repository for misc. scripts and other stuff:

There, the scripts and example data files are provided.


In this post I described a simple Python script used to generate iterative configs. The script is pretty simple and can be easily adapted for other similar tasks. I hope this script will be useful for other people as well.

Denis Borchev
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Denis Borchev

Engineer at Netcube LLC
I am a networking engineer, a geek and a generally nice person=)
Computer Networking Engineer with some experience; MSc Applied CS, CCIE #53271
Denis Borchev
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