yum update error repomd.xml

Hi,
my system is Red Hat Enterprise Linux Server release 6.5 (Santiago)
Linux srvvorelv01.avio.net 2.6.32-431.el6.x86_64 #1 SMP Sun Nov 10 22:19:54 EST 2013 x86_64 x86_64 x86_64 GNU/Linux

yum update
Loaded plugins: refresh-packagekit, rhnplugin, security
This system is receiving updates from ULN.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: rhel-x86_64-server-6. Please verify its path and try again

Can you help me

thanks