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

<stephane.litkowski@orange.com> Wed, 12 June 2019 08:23 UTC

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From: <stephane.litkowski@orange.com>
To: Xufeng Liu <xufeng.liu.ietf@gmail.com>, "lsr@ietf.org" <lsr@ietf.org>, NETMOD WG <netmod@ietf.org>
Thread-Topic: [Lsr] A question on the parameter overriding in draft-ietf-isis-yang-isis-cfg
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Date: Wed, 12 Jun 2019 08:23:08 +0000
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Subject: Re: [Lsr] A question on the parameter overriding in draft-ietf-isis-yang-isis-cfg
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Hi Xufeng,

Good catch, I think there is a mistake here, the expected behavior is the one described in the draft. We should not use a default value for the level-x leaves.
Will fix it in the next release as part of the AD review.

Brgds,

Stephane


From: Lsr [mailto:lsr-bounces@ietf.org] On Behalf Of Xufeng Liu
Sent: Sunday, June 09, 2019 16:35
To: lsr@ietf.org; NETMOD WG
Subject: [Lsr] A question on the parameter overriding in draft-ietf-isis-yang-isis-cfg


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



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