When you add new repo's you run the risk of your main repo's being overwritten with faulty stuff that can destabilze your servers to prevent this you can install protectbase. This will prevent those repo's being overwritten by other repo's keeping your main intact. Shh to your server and execute:
Check if /etc/yum/pluginconf.d/protectbase.conf has been set to enabled. Then you have to start editing repo files /etc/yum.repos.d/ and add protect=1 to the ones you want to protect like so:
yum install yum-plugin-protectbase
Note of caution, every repo where protect is not added will be treated as protected.
[base] name=CentOS-$releasever - Base mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os #baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/ gpgcheck=1 gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos4 protect=1