The checksum type used in repomd.xml and for packages in the metadata can be sha256 or older sha1 or sha. The default is now “sha256″, all RHEL 6 or Centos 6 kernels use sha256 by default when createrepo is run. If your repository was created on a RHEL 6 system and a RHEL 5 client is running a yum install, this is where the problem will arise. The older yum version 3.0 doesn’t support the sha256 checksum and can only support sha, and not sha1. Yum 3.0 must use sha. So the fix is simple, try to create your repo using sha instead of the default sha256
follow below steps to fix the issue
#cd /repository path (/var/pub/ftp/rhel6/)
#createrepo -s sha .
#yum clean all
#yum repolist all