NANOG Posting on Configuration Analysis

Terry Slattery
Principal Architect

The North American Network Operators Group (NANOG) email list just had an interesting exchange about  configuration management.  Someone asked:

For auditing, is there any Cisco Router/Switch configuration analysis tool?

Someone else replied:

Depends on your goal.
Trying to identify and fix configuration problems?
Take a look at NetMRI from Netcordia.
We demo’d it. We liked it and bought it.

There are two types of configuration problems:

  1. Configuration changes that are incorrect.
  2. Configurations that don’t match your configuration policies.

I’ll cover the first of these here and follow up with another post about the second.

Since 40%-60%* of network problems are due to configuration mistakes, checking device configurations is critical to network uptime.  And when you have an outage, the most likely cause is network configuration change.  Obviously, tracking configuration changes can have a big impact on network reliability.  What kind of tracking is useful in real networks?

I like to have a configuration repository where all past configurations are kept.  Keeping the last few copies isn’t sufficient when someone is doing a config debug scenario and will blast through ten or fifteen config changes in an hour. Keep them all; disk space is cheap!  Of course, you need a way to organize them so you can quickly identify which configurations to compare when looking for a change.  Once you’ve identified two configs to compare, use a side-by-side contextual diff display that colors the changes, as shown in the figure below.


How do you prevent the incorrect configuration from being fielded?  Some organizations use a change control board and peer review process to review the configurations before they are fielded.  Configuration control like this is a sign of a more mature organization, provided the process is followed (having a process and not following it has the opposite sign).   If you have known configuration policies for each network building block module, then you should be able to compare your validated configuration templates against the change to determine if there is a deviation from policy that should be flagged and investigated further.



Re-posted with Permission 

NetCraftsmen would like to acknowledge Infoblox for their permission to re-post this article which originally appeared in the Applied Infrastructure blog under


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