Solstice Enterprise Agents 1.0 User Guide

Chapter 3 SNMP-Based Master/Subagent

3.1 SNMP-Based Master/Subagent Overview

The Master Agent is the main component of Solstice Enterprise Agent technology. It runs as a daemon process and listens to User Datagram Protocol (UDP) port 161 for SNMP requests. The Master Agent also opens another port to receive SNMP trap notifications from various subagents. These traps are forwarded to various managers, as determined by the configuration files.

3.1.1 Invoking the Master Agent

A system startup script file invokes the Master Agent when the system is initially booted. Upon the Master Agent's invocation, it reads its various configuration files and appropriate actions are performed by activating subagents, determining the subtree OID for various subagents, populating its own MIBs. The Master Agent provides the following functionality:

Note -

If you have snmpd (part of Domain Manager) or some other SNMP agent using port 161 running, the Solstice Enterprise Agents cannot run.

3.1.2 Invoking the Subagent

A subagent can be invoked in the following ways:

3.1.3 Communicating With the Subagent

Any communication from subagents to the Master Agent is done through UDP port 161. The topic of sending traps to the Master Agent from the subagents is discussed in "3.1.6 Trap Notification".

Note -

The Master Agent communicates on separate ports for each subagent.

The Master Agent also checks to be sure that registered subagents are up and running, based on the following conditions:

3.1.4 Registering the Subagent

To register the subagent, the Master Agent binds it to the MIB. The Master Agent then determines its present location, using one of the following methods:

The binding policy relates to the registration of SNMP object identifiers (OIDs). It involves decision-making on the part of the Master Agent when dispatching SNMP requests to various subagents. The Master Agent supports the binding policy, as shown in Table 3-1.

Table 3-1 Binding Policy

Type of Registration 

Method of Registration 

Individual variable registration 

A subagent can manage individual variables 

Row registration 

A subagent can manage each row or multiple rows 

Table registration 

A subagent can register full and partial tables; partial table registration means that some columns of a table can be registered; for example, if a table has columns c1-c5, a subagent can then register a partial table managing c3 and c5 columns (only) of that table 

Duplicate registration 

NOT allowed 

Overlapping registration 

In the case of overlapped registration, the Master Agent dispatches requests on the basis of best OID match 

3.1.5 Sending Requests

The Master Agent supports the forwarding of SNMP requests (Get, Get Next, and Set) in two modes. The mode is indicated by providing an optional argument with the command-line invocation. These modes are:

Following is a list of allowable send requests.

3.1.6 Trap Notification

The subagents send the traps to the Master Agent; the Master Agent the decides which managers will receive the trap. This decision is configurable.

3.2 Description of the Subagent

Subagents do not interact with the network manager directly. Instead, they communicate with the Master Agent. The management responsibility of the subagent can be offered to the Master Agent; however, it is up to the Master Agent to accept or ignore the offers.

After the subagents have registered with the Master Agent, they wait for the SNMP requests from the Master Agent. If a request is received, it sends the appropriate response. Additionally, the subagents can send SNMP traps.

The subagent is composed of four major components: the SNMP driver, Service API stack, subagent application, and MIB database. The Master Agent is responsible for receiving and sending the SNMP Protocol Data Units (PDUs). It also encodes and decodes the PDUs. The SP (Stack) dispatches the received requests appropriately or calls the applicable call back functions.

The system interface module is the implementation of the call back) of all the variables managed by the subagent. The MIB compiler automatically generates this information.

Except for the system dependent interface and the MIB database, the other components are reusable by other subagents and are provided in a library.

3.2.1 Establishment

This step involves the effort by the subagents to announce their presence and describe how to communicate with them. The transport used is UDP. The port on which the subagents listen for the SNMP request is configurable, as mentioned in "4.4 Agents Access Control File" on "4.4 Agents Access Control File". This establishment can also be done dynamically through a handshaking protocol.

3.2.2 Maintenance

The subagent periodically checks for existence of the Master Agent. The dynamic subagents (not invoked by the Master Agent) periodically determine whether the Master Agent is active. The runtime library reregisters the subagent's subtree with the Master Agent in the event of any interruptions.

3.2.3 Termination

Termination involves the effort to inform the Master Agent that the dynamic agent is about to exit. The Master Agent can then remove the row entry from its subagent table in the memory.

3.3 Using the Master Agent

snmpdx [-h] [-p port_number] [-r filename]

[-a filename] [-c dirname] [-i filename] [-o filename] [-y]

[-m GROUP|SPLIT] [-d debug_level] 

The command line arguments are shown in Table 3-2.

Table 3-2 Master Agent Command Line Arguments



-a filename

A full path (default) of the access control file is: /etc/snmp/conf/snmdx.acl; see "4.4 Agents Access Control File" for more information

-c dirname

The full path of the (default) directory containing the agent resource files; the default directory is: /etc/snmp/conf

-d debug_level

Used for debugging purposes; depending on the debug_level (0-4), it prints a specific amount of information; the default debug_level is 0


Command usage 

-i filename

The full path of the PID used by the Master Agent for recovery after a crash; it contains tuples of the UNIX process ID, port number, resource name, and agent name (default files) is: /var/snmp/

-o filename

This file contains the tuple (enterprise_name, oid); for example, (Sun Microsystems,; the file (default) is used as a base for lookup in the trap-filtering and forwarding process is: /etc/snmp/conf/enterprises.oid


The forwarding of SNMP requests mode; the default is GROUP; see "3.1.5 Sending Requests"for a description of the two modes

-p port_number

The port number; the default port number is 161; for example, -p 1234

-r filename

The full path of the resource file name used by the Master Agent; stores information about the subagents that the Master Agent invokes and manages; the default resource file is /etc/snmp/conf/snmpdx.rsrc; see "4.2 Agents Resource Configuration Files"for more information


A recovery indicator signals when invoking the Master Agent process and then invokes the recovery module; the recovery process discovers which subagents in the previous session are still active; those subagents not active are re-spawned by the Master Agent