Hi Robert,

Thank you for the detailed explanation.
On the same lines that you mentioned, maybe some vendors were showing the 
loopback interface, as part of running. Once they implement NMDA, is it the 
intention of the authors to mandate that such configurations must be moved to 
"system" ? Currently they may have limitations on such interfaces and donot 
support deletion of such interfaces.

With Regards,
Rohit R Ranade

From: Robert Wilton [mailto:rwil...@cisco.com]
Sent: 17 May 2018 15:42
To: Rohit R Ranade <rohitrran...@huawei.com>; netmod@ietf.org
Subject: Re: [netmod] NMDA System controlled resource

Hi Rohit,
On 17/05/2018 10:30, Rohit R Ranade wrote:
Hi Robert,

So first , we try to get to know the system configuration.
Then for the configuration leaves (based on description), check whether system 
configuration trumps the intended configuration ? If yes, retain system 
configuration, Else apply intended configuration.
I think that this is probably an implementation choice, so my comments below 
are subjective.

E.g. I think that Junos devices always instantiate a loopback interface (lo0) 
even if not configured, but IOS XR does not.  This is fine, this is just a 
difference in architecture.

However, for both types of devices, configuring an IP address on the loopback 
interface should work just fine:

In the Junos case the lo0 interface already exists in <operational> with origin 
"system", along with an IP address underneath it with origin "intended".

In the XR case, both the loopback0 interface and IP address are configured, 
hence when the config is applied both data nodes appear in <operational> with 
the origin "intended".

Hence normally  it is up to the device implementation to decide whether a 
particular item of system configuration trumps the intended configuration.  
Whatever the system decides the appropriate value appears in <operational> and 
the origin (if supported) of that value in <operational> MUST indicate where it 
came from.  So in the general case, I wouldn't expect YANG modules to need to 
refer to system configuration.  However, there are some specific cases where it 
is useful to do so (e.g. RFC8343 describes system-controlled interfaces).

If for some leaf, there is no <intended> configuration , then apply system 
configuration .
For the systems that I work on then I would normally expect an explicitly 
configured value to trump a system value.   If the device does not allow values 
other than the system provided value then ideally it should deviate the data 
node to only allow the system assigned value to be configured.

If it is a container/list/etc then you may well need to merge the data coming 
from <intended>, system and other places as well (e.g. IP addressed learned via 


Is my understanding correct ?

With Regards,
Rohit R Ranade

From: Robert Wilton [mailto:rwil...@cisco.com]
Sent: 17 May 2018 14:29
To: Rohit R Ranade <rohitrran...@huawei.com><mailto:rohitrran...@huawei.com>; 
Subject: Re: [netmod] NMDA System controlled resource

Hi Rohit,

Section 5.3.2 states that you allowed to have configuration in 
<running>/<intended> for resources that could be present on the device but are 
not currently present.  The canonical example would be interface configuration 
for an interface on a linecard that isn't operational (either because it isn't 
present, or hasn't completely initialized).

Section 5.3.3 is saying that if the linecard becomes operational, then it may 
instantiate system controlled entries (in <operational>) for those interfaces.  
It also states that if there also happens to be configuration in 
<running>/<intended> for those interfaces then that configuration will also get 
applied as those interfaces as instantiated in <operational>.  All of the 
configuration that has been successfully applied would also appear in 


On 17/05/2018 04:57, Rohit R Ranade wrote:
Hi All,

RFC 8342 has below statement in Section 5.3.3
"If a system-controlled resource has
   matching configuration in <intended> when it appears, the system will
   try to apply the configuration; this causes the configuration to
   appear in <operational> eventually (if application of the
   configuration was successful).
Why does application of configuration for system-controlled resources depend on 
whether <intended> has configurations for that resource ? The configuration 
will still get applied as part of "system" configuration as shown in examples 
in Section C.1 in the same RFC given below

"In addition to filling in the default value for the auto-negotiation
   enabled leaf, a loopback interface entry is also automatically
instantiated by the system.  All of this is reflected in

With Regards,
Rohit R Ranade


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