In Figure 4.1, the SNMP agent is assumed to be part of the managed element. In the real world there are resources that either are not managed by a computer process or are managed using a non-SNMP protocol. Still many device manufacturers either do not provide support for SNMP or they may provide limited SNMP support. The latter includes switch vendors that have SNMP agents on some of their interface cards, but do not have agents that monitor the entire switch, including power supplies, the CPU, and the configuration of the entire switch. In this case if you want to use SNMP to manage the switch, you might need a so called ``proxy agent'' to monitor and control the switch. Such proxy agents allow access to remote information about passive components (cables, transceivers, repeaters) and resources of other protocol environments.
Figure 4.3 depicts an example for such a scenario. The protocol between the SNMP proxy and the elements management system depends on the specifications of the latter, and usually will be a proprietary one.