This KB describes the behavior of the em0 interface when an optical SFP is used on the management port in MX10008 and MX10016.

The Routing Engine of the MX10008 and MX10016 supports the use of an optical SFP on the management port.
As shown in the configuration example below, the em0 interface is used for management port settings.
On MX10008 and MX10016, the Junos/VM host architecture is used. Since em0 is an interface on the VM guest (Junos), this can sometimes lead to confusion.

Example

groups {
    re0 {
        system {
            host-name TEST-re0;
        }
        interfaces {
            em0 {
                description TEST-re0-em0;
                unit 0 {
                    family inet {
                        address XXX.XXX.XXX.XXX/XX;
                    }
                }
            }
        }
    }
    re1 {
        system {
            host-name TEST-re1;
        }
        interfaces {
            em0 {
                description TEST-re1-em0;
                unit 0 {
                    family inet {
                        address XXX.XXX.XXX.XXX/XX;
                    }
                }
            }
        }
    }
}

If you attempt to bring down the link between the management port and the connected peer device by applying a disable configuration to em0, the interface on the peer device will remain in a physically link-up state.
In other words, the peer device will still be receiving light.
This is because em0 is an interface on the VM guest side.
If you want to physically bring the peer device interface to a link-down state (i.e., stop the optical signal), the following command is useful:

set vmhost management-if disable

This command disables the management port on the VM host side, causing the peer device to stop receiving the optical signal and transition to a physically link-down state.

This is expected behavior and not a problem.