Assume a trust is setup between Domain Y and Domain X

From the Domain Y DC issue the following command

 ldifde -f output.ldf -s {FQDN of DCx} -b {user account} {domain}  {password}   Connecting to "FQDN of DCx" Logging in as "user account" in domain "domain" using SSPI

Exporting directory to file output.ldf

Searching for entries...

Writing out entries............................................

3945 entries exported

The command has completed