In-band Management Network
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.
Layer 3 between PoP's
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.