Hi Stefan,
Yes, your JNCIS-ER fulfills the prerequisite requirement for both the JNCIS-ENT and the JNCIP-ENT while it is in the Active (Certified) and Inactive statuses.
Since you already hold a JNCIS-ER, you can go directly for the JNCIP-ENT., If you earn the JNCIP-ENT, you are automatically granted a JNCIS-ENT, instead of your JNCIS-ER being renewed, as this is a legacy certification that is being phased out.
Please, visit our website for more information about the recertification process at http://www.juniper.net/us/en/training/certification/already-certified/recertification/.
Regards,