How to enable epel repository and yum update in redhat 7.4

# sudo rpm -ivh https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
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#yum update
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# yum install transmission
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 # yum repolist
Loaded plugins: langpacks, product-id, search-disabled-repos, subscription-manager
This system is not registered with an entitlement server. You can use subscription-manager to register.
repo id                                               repo name                                                                                 status
epel/x86_64                                           Extra Packages for Enterprise Linux 7 - x86_64                                            12,123
repolist: 12,123

# cd /etc/yum.repos.d/
# ls
epel.repo  epel-testing.repo  redhat.repo





how to clean yum list
#yum clean all
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#rm -rf /var/cache/yum
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#yum update