This paper describes the standard or testbed-agnostic interfaces offered by Community-Lab an exemplifies its use in a reference experiment, the Island Connectivity Experiment. The experiment provides a web application that shows a matrix of network measurements across a slice of testbed nodes from different community networks in the testbed. The experiment uses testbed-agnostic and interoperable interfaces such as the Slice-based Facility Architecture (SFA) for resource management, resource description (RSpec), experiment management framework (OMF), instrumentation and measurement collection (OML), and presents a web interface. The experiment uses SFA to selects nodes for a slice, runs the measurements using OMF and collects measurements using OML. The experience in running the experiment validates the implementation of these testbed-agnostic interfaces, shows the complementarity and correct integration among the tools and interfaces as part of the Fed4FIRE federation, and confirms the value of a federation of testbeds and the usage of testbed-agnostic tools for experimentation.