Simple Network Management (SNMP) send operation requests and responses as SNMP message. A Simple Network Management message consists of an SNMP protocol data unit plus supplementary message header units classified by the appropriate RFC. An SNMP agent sends data in two circumstances:
• When it reacts to a request from an SNMP manager.
• When a trap event takes place
Below mentioned are SNMP message types and their function:
• GET - Accesses and recover the present value of one or more MIB objects on an SNMP agent.
• GetResponse - Responses to a Get, GetNext, or Set function.
• GetNext – browses the complete tree of MIB objects, interpreting the worth of variables in the MIB in sequence. Generally, one use GetNext to get information from chosen lines from one or more rows of a chart. GetNext is particularly helpful for browsing dynamic charts, like n internal IP route chart or an ARP chart, reading through the chart one row at a time.
• GetBulk – recovers data in pieces as large as feasible inside the given limitations on the message volume. GetBulk, which accesses several values at one time without employing a GetNext message, lessens the amount of protocol exchanges needed to recover a big amount of data. To ignore fragmentation, limit the maximum message volume to a volume smaller compared to the path maximum transmission unit, the largest frame volume allowed for a sole frame on users’ network. Generally, when it isn’t recognized how many rows are in a table, GetBulk is employed to browse entire rows in the table.
• SET – alters the present value of an MIB object. To update a MIB value on the SNMP agent, the SNMP manager should have access to the object. SET is employed occasionally, since most MIB objects are read-only by default, thus illegal changes can’t be made.
• Trap – informs the particular SNMP manager when an unforeseen event takes place locally on the managed host. One can employ traps for constraint security checking or for troubleshooting.
SNMP uses the connectionless UDP (User Datagram Protocol) service to send SNMP messages. SNMP employs the simple UDP transport service, which assures neither delivery nor appropriate sequencing of delivered packet, thus SNMP can carry on functioning after several other network services have malfunctioned. By default, UDP port 161 is used to listen to SNMP messages, and port 162 to listen to SNMP traps. If essential, for instance, because your company already uses ports 161 and port 162 for several other protocols – you can modify these port settings by configuring the local services file.
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