This article provides a step-by-step procedure for installing a license file on an EX device that is running in virtual-chassis mode with graceful Routing Engine switchover (GRES) enabled.
If GRES is enabled, the license keys are synchronized on the backup Routing Engine (RE) after adding the license key on the primary RE.

If two REs are in the virtual chassis, you will need to add the license for both REs. But the license key will be effective on the primary RE. In order to upload the license key on the backup RE, the installation can be done from the primary RE. No RE switchover is required.

The installation operation is the same as for a normal single chassis. You only need to install the license for the backup RE from the CLI on the primary RE.

Steps for license installation for virtual chassis

1. Add the license for the primary RE.

user@switch>request system license add terminal

Copy-paste the license key for RE0.

Press “Control+D” to quit.

2. Verify the installed license:

user@switch>show system license

Check whether the feature name, serial number, and duration are as expected.

3. Add the license for the backup RE.

user@switch>request system license add terminal

Copy-paste the license key for RE1.

Press “Control+D” to quit.

4. Verify the installed license:

user@switch>show system license

Check whether the feature name, serial number, and duration are as expected.

user@switch> show system license
License usage:
                                 Licenses     Licenses    Licenses    Expiry
  Feature name                       used    installed      needed
  bgp                                   0            2           0    2021-09-06 05:30:00 IST   <<< Licenses installed counts 2 if it is "date-based" license;
  isis                                  0            2           0    2021-09-06 05:30:00 IST       Note: Licenses installed counts 1 if it is "permanent" license or Agile Licensing.
  mpls                                  0            2           0    2021-09-06 05:30:00 IST

Licenses installed:
  License identifier: EmergencyJUNOS048953532
  License version: 4
  Valid for device: TC3720010119
  Customer ID: Juniper Internal
  Features:
    Routing          - Advanced licensed routing protocols
      date-based, 2021-08-06 05:30:00 IST - 2021-09-06 05:30:00 IST　　<<<< "date-based" license in this example

  License identifier: EmergencyJUNOS440847173
  License version: 4
  Valid for device: TC3720110141
  Customer ID: Juniper Internal
  Features:
    Routing          - Advanced licensed routing protocols
      date-based, 2021-08-06 05:30:00 IST - 2021-09-06 05:30:00 IST

A permanent license example is given below:

user@switch> show system license
License usage:
                                 Licenses     Licenses    Licenses    Expiry
  Feature name                       used    installed      needed
  bgp                                   0            1           0    permanent <<< Licenses installed counts 1 if it is "permanent" license
  isis                                  0            1           0    permanent
  mpls                                  0            1           0    permanent

Licenses installed:
  License identifier: JUNOS162350XXX
  License version: 4
  Valid for device: TC37204XXXXX
  Customer ID: Juniper Internal
  Features:
    Routing          - Advanced licensed routing protocols
      permanent         <<<<<<<<<<<<<<<<<<<<<<<"Permanent" license example

  License identifier: JUNOS722234XXX
  License version: 4
  Valid for device: TC37204XXXXX
  Customer ID: Juniper Internal
  Features:
    Routing          - Advanced licensed routing protocols
      permanent

2023-09-15: Updated content; added note for license count of Agile licensing.
2021-08-30: Updated content; explained how "Licenses installed" is counted in case of "date-based" or "permanent" license. 
2019-04-24: Updated content. Both chassis licenses can be installed from the master RE.