ACX5048 is equipped with three management ports, two labeled as C0 and one C1 port. Of the two labeled as C0 ports, only the port with built-in 10/100/1000 Base-T SFP is enable to be used.
Second C0 port on ACX5048 platform is not enable
Management ports in ACX5048 platforms are mapped to em0 (C0) and em1 (C1) interfaces. When checking on the back of the ACX5048, you can see two different ports mark as C0; from which only the port with 10/100/1000 Base-T SFP is enabled for use. The second C0 port, located over the C1 port, is disable.
In the following document, you can see that the port marked with number (3) is the only C0 port that is enable. In ACX5000 series, only C0 with copper is supported/enabled . http://www.juniper.net/techpubs/en_US/release-independent/junos/topics/reference/general/acx5000-port-management-leds.html