[CentOS] Fwd: SOS

Greetings.

Please help me.  I have installed Centos 7 , but when I do,  yum update
 give me error next.


Complementos cargados:fastestmirror, langpacks


 One of the configured repositories failed (Desconocido),
 and yum doesn't have enough cached data to continue. At this point the only
 safe thing yum can do is fail. There are a few ways to work "fix" this:

     1. Contact the upstream for the repository and get them to fix the
problem.

     2. Reconfigure the baseurl/etc. for the repository, to point to a
working
        upstream. This is most often useful if you are using a newer
        distribution release than is supported by the repository (and the
        packages for the previous distribution release still work).

     3. Disable the repository, so yum won't use it by default. Yum will
then
        just ignore the repository until you permanently enable it again or
use
        --enablerepo for temporary usage:

            yum-config-manager --disable <repoid>

     4. Configure the failing repository to be skipped, if it is
unavailable.
        Note that yum will try to contact the repo. when it runs most
commands,
        so will have to try and fail each time (and thus. yum will be be
much
        slower). If it is a very temporary problem though, this is often a
nice
        compromise:

            yum-config-manager --save
--setopt=<repoid>.skip_if_unavailable=true

Cannot find a valid baseurl for repo: base/7/x86_64

Gracias de antemano.

Hugo