ZNDP 039 – Design Documents

Network Design, Podcast, ZNDP

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In Today’s episode, we are going to cover Design Documents.  I recently had a number of coaching sessions where design documents were discussed.  I then attempted to remember how I learned about all of the Design Documents that I have seen, written, and peer reviewed over my career, but I couldn’t find a good answer other than experience.  Well today we are going to shed some light on what I would call the most common Design Documents our there.  Lets get started.

Design Documents – Agenda

  • Customer Requirements Document (CRD)
  • Reference Architecture (RA)
  • High Level Design (HLD)
  • Low Level Design (LLD)

Design Documents – CRD

  • Customer Requirements Documents
    • Undervalued and skipped
    • Built together with the customer and the Designer
    • Illicit information from customer, thats our role as a Designer

Design Documents – RA

  • Reference Architectures
    • Abstraction of an Architecture in a perfect world situation
    • Sometimes you will see a HLD, a LLD, and a mix of these together.  Its not always end to end though and fully inclusive.  Its more like here is the 85% solution in a perfect world.  Now you as a Design need to apply this reference architecture within the environment you are working on with the associated requirements.

Design Documents – HLD

  • High Level Design
    • Specific to an environment
    • Apply information from CRD and associated Reference architectures to build out the HLD
    • Example??
    • You need a router here, a Firewall there, and some sort of transport connecting everything together.  
    • Things like Single points of failure, dual points of failure, and , shared fate would be discussed and ruled out 
    • Design specific decisions are highlighted and discussed.  For example, this document would discuss decisions around MPLS BGP Route Reflect Design.  Cold potato and hot potato routing.  If there should be dedicated RRs for IP address-families versus VPN address-families, or should they be mingled on the same set of BGP RRs.  More importantly, it would discuss the pros and cons to each of these design decisions.
    • You could also see an As-Built version of this as well, where someone documented in an HLD formation what the current environment is.
    • What you will not find here, is the actually configuration of features and devices. This aligns perfectly with the next design document, the LLD

Design Documents – LLD

  • Low Level Design
    • The Low Level Design Document, or LLD is where all of the technical configurations of features, devices, and all of those nerd knobs you want to use is going to be. 
    • Some times called Detail Design Documents. There are also As-Built versions of these as well depending on who is creating the documents.

 

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