Add support for IPv6 RA reachable-time and router-preference

[vishak-mut]

• (M) release/models/interfaces/openconfig-if-ip.yang
  • Add support for IPv6 RA reachable-time (RFC 4861).
  • Add support for IPv6 RA router-preference (RFC 4191).

Change Scope

This change adds support for configuring two standard IPv6 Neighbor Discovery (ND) Router Advertisement (RA) parameters within the OpenConfig interfaces IP model:

1. reachable-time (RFC 4861 §4.2 & §6.2.1):

   • Carried in the 32-bit Reachable Time field of ICMPv6 Router Advertisement messages.
   • Defines the time (in milliseconds) a node assumes a neighbor remains reachable after receiving a reachability confirmation.
   • Setting to 0 indicates that reachable time is unspecified by the advertising router.
   • Essential in bare-metal and data center environments with dynamic or routed host interfaces to tune neighbor unreachability detection (NUD) and ARP/ND cache refresh cycles.

2. router-preference (RFC 4191 §2.1 & §2.2):

   • Advertised in the 2-bit Default Router Preference (Prf) field of the flag octet in ICMPv6 Router Advertisement messages.
   • Allows routers to signal preference levels (HIGH, MEDIUM, LOW) to downstream hosts.
   • Multi-homed hosts utilize this preference to prioritize default gateway selection when multiple routers are present on a shared or redundant segment.

The change is backwards-compatible and purely additive (minor version bump 3.9.0 -> 3.10.0).

Platform Implementations

Proof of implementation across two separate network operating system platforms:

• HPE Aruba Networking (AOS-CX) documentation

  • AOS-CX supports both parameters under the interface context:
    • ipv6 nd ra reachable-time <TIME>: Configures the reachable time in milliseconds advertised in RAs (default: 0).
    • ipv6 nd router-preference {high | medium | low}: Configures the Default Router Preference in RAs (default: medium).
  • Verification: show ipv6 nd interface <interface-name>

• Arista EOS documentation

  • Arista EOS supports both parameters under interface configuration mode:
    • ipv6 nd reachable-time <0-3600000>: Configures the reachable time in milliseconds advertised in RAs (default: 0).
    • ipv6 nd router-preference {high | medium | low}: Configures the Default Router Preference in RAs (default: medium).
  • Verification: show ipv6 interface <interface-name>

Tree View

 module: openconfig-interfaces
   +--rw interfaces
      +--rw interface* [name]
         +--rw subinterfaces
            +--rw subinterface* [index]
               +--rw oc-ip:ipv6
                  +--rw oc-ip:router-advertisement
                     +--rw oc-ip:config
                     |  +--rw oc-ip:enable?              boolean
                     |  +--rw oc-ip:interval?            uint32
                     |  +--rw oc-ip:lifetime?            uint32
                     |  x--rw oc-ip:suppress?            boolean
                     |  +--rw oc-ip:mode?                enumeration
                     |  +--rw oc-ip:managed?             boolean
                     |  +--rw oc-ip:other-config?        boolean
+                    |  +--rw oc-ip:reachable-time?      uint32
+                    |  +--rw oc-ip:router-preference?   enumeration
                     +--ro oc-ip:state
                        +--ro oc-ip:enable?              boolean
                        +--ro oc-ip:interval?            uint32
                        +--ro oc-ip:lifetime?            uint32
                        x--ro oc-ip:suppress?            boolean
                        +--ro oc-ip:mode?                enumeration
                        +--ro oc-ip:managed?             boolean
                        +--ro oc-ip:other-config?        boolean
+                       +--ro oc-ip:reachable-time?      uint32
+                       +--ro oc-ip:router-preference?   enumeration

[gemini-code-assist]

Code Review

This pull request updates the openconfig-if-ip.yang module to version 3.10.0, introducing reachable-time and router-preference leaves to the IPv6 Router Advertisement configuration. The review feedback recommends adding a range constraint of 0..3600000 to the reachable-time leaf to comply with RFC 4861 limits.

[gemini-code-assist]

According to RFC 4861 Section 6.2.1, the ReachableTime value MUST be no greater than 3,600,000 milliseconds (1 hour). It is highly recommended to add a range constraint to the reachable-time leaf to enforce this limit and prevent invalid configurations.

    leaf reachable-time {
      type uint32 {
        range "0..3600000";
      }
      units milliseconds;
      default 0;
      description
        "The time, in milliseconds, that a node assumes a neighbor is
        reachable after having received a reachability confirmation.  Set
        to 0 to indicate that reachable time is unspecified by this router.";
      reference "RFC4861: Neighbor Discovery for IPv6, section 4.2 and 6.2.1";
    }