pc1 has a nexus yum proxy
pc2 has a nexus yum proxy to pc1
pc3 yum config direction pc2
error info:
failure: repodata/repomd.xml from extras: [Errno 256] No more mirrors to try.
http://114.132.40.107:8089/repository/yum-group/centos/7/extras/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found