A number of different hotplug events have been added to kernel 2.6. Every new driver can introduce a new event. Only events already known have agents assigned to them. These agents carry out the necessary actions.
The majority of device agents load kernel modules, but occasionally they also call additional commands. For example, with some computer architectures, such as IBM S390, a particular value must be entered for every device in procfs or sysfs, for that device to be initialized. Under SUSE LINUX, this is handled by /sbin/hwup or /sbin/hwdown. These programs search for a configuration suitable for the device in /etc/sysconfig/hardware. If /sbin/hwup does not find any configuration, modules are automatically loaded. For further information about this point, see Section 14.4. “Automatic Module Loading”. Further information about /sbin/hwup is contained in file /usr/share/doc/packages/sysconfig/README and in man hwup.
Interface agents perform two main tasks. First, they initialize the interface or call udev to create a device node. Second, network interfaces are initialized with /sbin/ifup and deactivated with /sbin/ifdown. Further details about this subject can be found in the file /usr/share/doc/packages/sysconfig/README and with man ifup. There is also a man page for udev (man udev). Another source of information is Section 15. Dynamic Device Nodes with udev.