def METHOD_NAME(self: SharedUtils) -> str | None:
    if self.underlay_router:
        if self.underlay_routing_protocol in ["isis", "isis-ldp", "isis-sr", "isis-sr-ldp"]:
            if self.mpls_lsr:
                default_isis_instance_name = "CORE"
            else:
                default_isis_instance_name = "EVPN_UNDERLAY"
            return get(self.hostvars, "underlay_isis_instance_name", default=default_isis_instance_name)
    return None