It is very simple and easy if you can access the RootCA,
Steps are -login to your ROOTCA and launch MMC for the CA.
This can be done by Right Click the “Revoked Certificates” – All Tasks – Publish
Copy the CRL file from the Root CA located under %systemroot%\system32\certsrv\certenroll to the Sub CA Server
Copy the above file to the InetPub folder (HTTP Path) in the Sub CA server which is by default located under the C:\inetpub\wwwroot\Certdata
Open an Admin Command Prompt and run the following command to publish it to the Active Directory (LDAP Path).
certutil -f -dspublish ” C:\Inetpub\wwwroot\certdata\RootCA.crl
Hope this helps.
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