If you are using SCOM 2012 to monitor network devices that means you have to use resource pools. In order to create resource pool for network device you have to change the setting of the other default resource pools (Notifications Resource Pool, AD Assignment Resource Pool and All Management servers resource pool). These polls have to be set from automatic membership to manual for a certain reasons. Find these reasons and how to change the settings of these resource pools in an article titled Automatic and Manual Resource Pools in Operations Manager 2012  by Cameron Fuller.