To allow the subagents to perform their functions, the SNMP agent binds to an arbitrarily chosen TCP port and listens for connection requests from subagents. A well-known port is not used. Every invocation of the SNMP agent potentially results in a different TCP port being used.

Agents, or SNMP servers, are responsible for performing the network management functions requested by the network management stations.

A subagent provides an extension to the functionality provided by the SNMP agent. You can use the subagent to define your own MIB variables, which are useful in your environment, and register them with the SNMP agent. When requests for these variables are received by the SNMP agent, the agent passes the request to the subagent and returns a response to the agent. The SNMP agent creates an SNMP response packet and sends the response to the remote network management station that initiated the request. The existence of the subagent is transparent to the network management station.

A subagent of the SNMP agent determines the port number by sending a GET request for an MIB variable, which represents the value of the TCP port. The subagent is not required to create and parse SNMP packets, because the DPI application programming interface (API) has a library routine query_DPI_port(). After the subagent obtains the value of the DPI TCP port, it should make a TCP connection to the appropriate port. After a successful socket connect() call, the subagent registers the set of variables it supports with the SNMP agent. For information about the connect() call, see the connect() call information in z/OS Communications Server: IP Sockets Application Programming Interface Guide and Reference. When all variable classes are registered, the subagent waits for requests from the SNMP agent.

If connections to the SNMP agent are restricted by the security product, then the security product user ID associated with the subagent must be permitted to the agent's security product resource name for the connection to be accepted. See the Simple Network Management Protocol (SNMP) information in z/OS Communications Server: IP Configuration Guide for more information about security product access between subagents and the z/OS® Communications Server SNMP agent.