[Lsr] A question on the parameter overriding in draft-ietf-isis-yang-isis-cfg

Xufeng Liu <xufeng.liu.ietf@gmail.com> Sun, 09 June 2019 14:35 UTC

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From: Xufeng Liu <xufeng.liu.ietf@gmail.com>
Date: Sun, 9 Jun 2019 10:35:11 -0400
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To: lsr@ietf.org, NETMOD WG <netmod@ietf.org>
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Subject: [Lsr] A question on the parameter overriding in draft-ietf-isis-yang-isis-cfg
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In Section 2.3. and many other locations, the current IS-IS model applies
the parameter overriding rule as below:

[Quote]:

2.3 <https://tools.ietf.org/html/draft-ietf-isis-yang-isis-cfg-35#section-2.3>;.
Per-Level Parameters


   Some parameters allow a per level configuration.  In this case, the
   parameter is modeled as a container with three configuration
   locations:

   o  a top-level container: corresponds to level-1-2, so the
      configuration applies to both levels.

   o  a level-1 container: corresponds to level-1 specific parameters.

   o  a level-2 container: corresponds to level-2 specific parameters.

               +--rw priority
               |  +--rw value?     uint8
               |  +--rw level-1
               |  |  +--rw value?   uint8
               |  +--rw level-2
               |     +--rw value?   uint8

   Example:

           <priority>
               <value>250</value>
               <level-1>
                   <value>100</value>
               </level-1>
               <level-2>
                   <value>200</value>
               </level-2>
           </priority>

   An implementation SHOULD prefer a level specific parameter over a
   level-all parameter.  As example, if the priority is 100 for the
   level-1, 200 for the level-2 and 250 for the top-level configuration,
   the implementation should use 100 for the level-1 and 200 for the
   level-2.

[End of Quote]


In the model, all three value leaves above have a default statement
“default 64”, which brings up my question for the following example:


           <priority>
               <value>250</value>
               <level-1>
                   <value>100</value>
               </level-1>
           </priority>


The user does not provide a configured value for level-2. According to
Section 7.6.1. of RFC7950, because the default value is in use, “the server
MUST operationally behave as if the leaf was present in the data tree with
the default value as its value”. This means the priority value for level-2
will be 64 (the default value), so the value 250 can never take effect as
intended in the above quoted Section 2.3.


Is my understanding correct?


Since this is a generic question, I am CC’ing NETMOD WG too.


Thanks,

- Xufeng