CentOS-6.1 is based on the upstream release EL 6.1 and includes packages from all variants. All upstream repositories have been combined into one, to make it easier for end users to work with.

All machines running CentOS-Release-CR on 6.0 would already be running code that is included in the 6.1 install media. We highly recommend everyone use the CR repository in order to get all bugfix and security
updates as soon as possible. More information on the CR repository can be found at : http://wiki.centos.org/AdditionalResources/Repositories/CR

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Upgrading from CentOS-4 or CentOS-5:

We recommend everyone run through a reinstall rather than attempt an inplace upgrade from CentOS-4 or CentOS-5