DRAFT, more detail to be added...

  
vi /etc/yum.repos.d/centos-base.repo
vi /etc/yum.repos.d/centos.repo
yum --enablerepo centos.repo
yum update
grep enabled /etc/yum.repos.d/ # only 1 can/should be enabled

sudo rpm -Uvh http://download.fedoraproject.org/pub/epel/5/i386/epel-release-5-4.noarch.rpm

query package content of a repository package

  
repoquery --plugins -lq net-snmp

yum clean all
yum update