"Repodata is over 2 weeks old. Install yum-cron? Or run: yum makecache fast" というメッセージがログに記録されるのはなぜですか?

Issue

• "Repodata is over 2 weeks old. Install yum-cron? Or run: yum makecache fast" というメッセージがログに記録されるのはなぜですか?

root@bxxx:[~]# yum upgrade
Loaded plugins: langpacks, product-id, subscription-manager
Repodata is over 2 weeks old. Install yum-cron? Or run: yum makecache fast
epel/x86_64/metalink                                                                                                   | 8.4 kB  00:00:00     
epel                                                                                                                   | 3.7 kB  00:00:00     
rhel-7-server-rpms                                                                                                     | 3.7 kB  00:00:00     
(1/2): epel/x86_64/primary_db                                                                                          | 2.3 MB  00:00:01     
(2/2): rhel-7-server-rpms/7Server/x86_64/primary_db