Add hierarchical-fec bool leaf to static nexthop-group

[romeyod]

Change Scope

• Add hierarchical-fec bool leaf to Static nexthop-group to control hierarchical forwarding equivalence class (FEC) behavior on next-hop groups:
  /network-instances/network-instance/static/next-hop-groups/next-hop-group/config/hierarchical-fec
/network-instances/network-instance/static/next-hop-groups/next-hop-group/state/hierarchical-fec
  When True: The nexthop-group preserves its hierarchical structure, explicitly pointing to the resolving NHG for each next-hop.
  When False: The nexthop-group contents are flattened, or a single random next-hop is selected.
• This change is backwards compatible

Context:
Add hierarchical-fec bool leaf to Static NHG

Tree View

module: openconfig-network-instance
  +--rw network-instances
     +--rw network-instance* [name]
           +--rw static
           |  +--rw next-hop-groups
           |  |  +--rw next-hop-group* [name]
           |  |     +--rw name         -> ../config/name
           |  |     +--rw config
           |  |     |  +--rw name?   string
  +        |  |     |  +--rw hierarchical-fec?   boolean
           |  |     +--ro state
           |  |     |  +--ro name?   string
  +        |  |     |  +--ro hierarchical-fec?   boolean
           |  |     +--rw next-hops
           |  |        +--rw next-hop* [index]
           |  |           +--rw index     -> ../config/index
           |  |           +--rw config
           |  |           |  +--rw index?   -> ../../../../../../next-hops/next-hop/index
           |  |           +--ro state
           |  |              +--ro index?   -> ../../../../../../next-hops/next-hop/index

[romeyod]

@nupkanoi for visibility

[gemini-code-assist]

Code Review

This pull request updates the openconfig-network-instance-static YANG module to version 0.3.0 and introduces a new hierarchical-fec boolean leaf to the static next-hop-group configuration. Feedback indicates a logical inconsistency in the leaf's description, which references next-hop-group pointers that are currently missing from the schema. It was also suggested to define a default value of false for the new leaf to maintain backward compatibility and ensure deterministic behavior.

[gemini-code-assist]

The description for hierarchical-fec refers to next-hop entries "explicitly pointing to the resolving next-hop-group" and mentions a "referenced group". However, the current static next-hop configuration (defined in static-nhg-next-hop and oc-loc-rt:local-static-nexthop-config) does not provide a mechanism for a next-hop to reference another next-hop-group.

To enable the hierarchical behavior described, a leaf (e.g., next-hop-group as a leafref) should be added to the next-hop configuration. Additionally, there is a discrepancy between the PR description ("single random next-hop") and the leaf description ("single next-hop... from each referenced group").

[gemini-code-assist]

It is recommended to provide a default value for boolean leaves in OpenConfig to ensure deterministic behavior when the leaf is not explicitly configured. Based on the description, false (representing flattened behavior) appears to be the appropriate default for backward compatibility.

      type boolean;
      default false;

[robshakir]

@dplore @navaneethyv @ElodinLaarz -- can we please make sure that we review this carefully with other vendors since this sounds like it has the risk of being very platform specific.

[romeyod]

@rgwilton @earies for other vendor inputs

[rgwilton]

We have discussed this internally and we believe that this behaviour should be managed by the system without requiring explicit user configuration. I.e., this feels too implementation (and perhaps hardware) specific. E.g., certainly the implication that if you don't set this flag then that would mean that you wouldn't automatically be able to make these hierarchical would be unhelpful.

So, in conclusion we are not supportive of this change.