"--ignore-externals" option will be used with svn checkout, svn update commands to disable externals definition processing.

More information can be found here.
Note: there is the potential to leverage svn:externals to gain read access to the entire Subversion repository. This can happen if you follow the normal practice of giving Jenkins credentials with read access to the entire Subversion repository. You will also need to provide the credentials to use when checking/polling out the svn:externals using the Additional Credentials option.