We have a network of around 100 clients running RHEL and one machine hosting a mirror of RedHat's official repository (via rsync). The clients point to the server to get their updates (via .repo file), but currently I have to manually go into each one and execute
$yum check-update
$yum list updates
$yum clean all
$yum update
The idea is to have the server do a sort of "push updates" and all the clients receive and apply the corresponding updates on their own.
There are many solutions to this; which one works will depend a lot on the specifics of your environment.
Other posters have already mentioned Saltstack and Ansible. There is also Puppet, a simple cron job on each of your servers, or Red Hat Satellite, just to name the three additional options I can think of first. Do note, however, that Red Hat Satellite can be a rather expensive option, so it may not fit your budget. A no-entry-cost alternative to it might be Spacewalk, which would give you the added advantage of letting you manage non-RHEL clients.
Overall, as Bananguin said, there are far too many answers to enumerate, and the applicability of those answers are are too dependent on your environment to discuss rationally in this forum.
yum itself cannot do this. You need some tool to remote control your 100 clients. To accomplish this there are probably more viable solutions than this site has users.
I'd suggest remote management using Saltstack. You need to setup one server as a master and, for ease of use, give him the DNS name salt in your domain. You then install every client as a minion. When doing so the minions register with the master. On the master you can bulk accept all clients and then use salt '*' pkg.upgrade to make your clients perform a yum upgrade.
For installation details and usage see the fine manual.
Bananguin advises salt for this task, but I believe that ansible is easier to setup and operate, especially for one-off uses like this.
