Your Enterprise Recon Master Server is in a corporate environment which may only access external resources (e.g mirror.centos.org & updates.groundlabs.com repos) via a proxy.
Add your corporate proxy into YUM's configuration.
To configure proxy settings for YUM:
Edit the /etc/yum.conf file and add the proxy details into the [main] stanza
Skip username & password if your proxy doesn't require one.
To check if it's working:
[root@er-master ~]# yum repolist
All information in this article is accurate and true as of the last edited date.