If above article doesn't help to resolve this issue please use https://bugs.centos.org/.

 One of the configured repositories failed (CentOS-6 - Base - 163.com),
 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. Run the command with the repository temporarily disabled
            yum --disablerepo=base ...

     4. Disable the repository permanently, 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 base
        or
            subscription-manager repos --disable=base

     5. 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=base.skip_if_unavailable=true

failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try.
http://mirrors.163.com/centos/6/os/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found
[root@n1 ~]# 
 

查看有无 yum安装包 ，有的话卸载：

查看：rpm -qa |grep yum

卸载：rpm -aq|grep yum|xargs rpm -e –nodeps

变更

rpm -qa|grep yum|xargs rpm -e --nodeps

http://mirrors.163.com/centos/7/os/x86_64/Packages/

wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.31-52.el7.noarch.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/python-iniparse-0.4-9.el7.noarch.rpm

还有几个自己下载

.下载yum源

wget http://mirrors.163.com/.help/CentOS7-Base-163.repo
# 复制下载的文件到配置目录下
$ cp CentOS7-Base-163.repo /etc/yum.repos.d/ 
# 切换到配置目录下
$ cd /etc/yum.repos.d/ 
# 重命名
$ mv CentOS-Base.repo CentOS-Base.repo.bak 
# 重命名
$ mv CentOS7-Base-163.repo CentOS-Base.repo

$ yum clean all 

$ yum makecache 

$ yum update