Here resource, optional and field terms seems to create misunderstanding. All of the representation of the resource appears needs to be updated, although optional appears "not need to be specified", with PUT command it takes all of the unspecified optional fields to default value. (which means erasing the configuration previously created.)
PATCH command provides partial updates to a specific field, could not find on PATCH on NSX Edge to update a single BGP neighbor or whether PATCH is supported on current NSX API, there are not specific examples about usage of PATCH for specific tasks done through REST API calls to NSX Manager.
These links could be helpful about:
http://wahlnetwork.com/2015/07/13/patch/
Read the Docs has a clever little blurb on PUT vs PATCH. Both methods are shown to “update a resource” in the semantics chart. The difference? PUT updates the entire resource while PATCH updates only the resources fields provided in the call. With PUT, I need to provide data for every field that the resource contains, otherwise the missing fields are set to null or the request fails. These are annoyances I’ve hit with the VMware NSX API on a number of occasions when wanting to update specific fields contained within an NSX Edge resource. The PATCH method sounds useful for updating one of a resource’s fields, such as the password field, without having to first GET all of the other fields.
https://restful-api-design.readthedocs.io/en/latest/methods.html#patch-vs-put
https://stackoverflow.com/questions/19732423/why-isnt-http-put-allowed-to-do-partial-updates-in-a-re...
So, based on RFC 7231, PUT should be used only for complete replacement of a representation, in an idempotent operation. PATCH should be used for partial updates, that aren't required to be idempotent, but it's a good to make them idempotent by requiring a precondition or validating the current state before applying the diff. If you need to do non-idempotent updates, partial or not, use POST. Simple. Everyone using your API who knows how PUT and PATCH works expects them to work that way, and you don't have to document or explain what the methods should do for a given resource. You're free to make PUT act in any other way you see fit, but then you'll have to document that for your clients, and you'll have to find another buzzword for your API, because that's not RESTful.
https://www.infoworld.com/article/3206264/application-development/how-to-perform-partial-updates-to-...
The HTTP PATCH method should be used whenever you would like to change or update just a small part of the state of the resource. You should use the PUT method only when you would like to replace the resource in its entirety. Note that PATCH is not a replacement for PUT or POST, but just a way of applying a delta update to a resource representation. Roy Fielding, who authored the REST architectural style and many web standards, said that he created PATCH because “partial PUT is never RESTful.”
