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Day(-1),Day-0,Day-1,Day-2..N Configurations

Cisco Employee

In the world of network automation, network function virtualization and cloud deployment it very common to come across terms like Day(-1) "Day-Minus-One", Day-0 "Day-Zero", Day-1 "Day-One", Day-2 "Day-Two" and Day-N "Day-N". This terms are widely used but not well defined, sometimes leading to misunderstanding and confusion.  This blog post is an attempt to arrive at an common understanding and definition of these terms. (feedback and comments are welcome).


Device configuration can be classified as Day(-1), Day-0, Day-1 and Day 2..N based on when they are applied to the device.

Conventions /Best-Practices/Guidelines/Recommendations:

Technically, the classification of configuration into Day-0,1,2,N is based on when the configurations is applied rather than what the configuration is trying to do.

The terms Day-0,1,2, etc. are also used in IT operations and other cloud operations domain. In these domains the classification of configurations into Day (-1),0,1,2,N the definitions vary slightly.

To align with the "general" understanding of the terms Day(-1),0,1,2,N, we recommend the following conventions.

  1. Initial configuration typically applied on power-on, boot-up.
  2. Information entered on serial consoles, buttons/switches on device
  3. Configurations that need knowledge of physical connectivity, typically entered by operators on the truck-roll
    1. ID of the Interface connected to the switch for example (assuming there is not automatic topology discovery method)
  4. Configurations that are considered per-requisite.
Day-0Minimum configuration required to establish communication between device and service orchestration system.
Day-1Configuration pushed by service orchestration system that are common to all Service instances. (example, NTP, Syslog, SNMP Trap destination etc.)

On-Going configurations pushed on the device for day-to-day operations. Configurations pushed to realize one instance of a service

Cisco Employee

This is great. Note that day-1 and day-2/n apply equally to physical devices as to VNFs, and require a NED. Day-0 applies only to VNFs, and it is applied via the VNFM, e.g. ESC (from an NSO perspective - obviously physical devices need bootstrap config too, but NSO cannot reach the device before it has a bootstrap)

Note also that we tend to focus on day-2/n with NSO. However, NSO is also excellent at managing day-1 configuration, with device templates and compliance reporting. In fact, if you look at what Cisco IT has done, this is almost exclusively focused on day-1.

Cisco Employee

"Day-0 applies only to VNFs" - How are we categorising configuration pushed via PnP to a PNF?  I had understood that PnP is really just another mechanism to get a Day-0 configuration onto a PNF.

Cisco Employee

Good point! ☺