Every IXP device is usually reachable on an IP address, so that the NOC can use ssh / telnet or other means to maintain and configure the device. These IP addresses would typically be placed into the services DMZ, and are not reachable from the outside world.
It is possible to route management traffic in case an IXP also has routing infrastructure next to its switching-infrastructure. This is not mandatory and can also be done with a Layer 2 VLAN, described in the next point.
Management can also be spanned between the IXP switches with a single VLAN. This is a rather common and cheap approach, as no additional routing-hardware or dedicated lines are required, and the existing IXP Infrastructure can be used. Management usually has very low bandwidth needs, so this is usually no issue on interconnected tagged ports between IXP switches.