The index value which uniquely identifies the local interface through which the next hop of this route should be reached. The interface identified by a particular value of this index is the same interface as identified by the same value of ipv6IfIndex. For routes with global address next hop this value can be zero.
Sources for this description
Netdisco MIB archive copiesNETGEAR official product and EOL MIB downloadsNETGEAR MIB copies (LibreNMS/Observium Community)