SNMP Protocol

The management station or SNMP manager uses the SNMP protocol to collect objects information in the SNMP agent.

 We could divide the SNMP protocol into three parts :

• The SNMP protocol itself, which defines how to exchange data, the packet formats, the different types of requests, the security mechanisms.
• The information structure contained in the SNMP agent and the storing methods. These rules come form the SMI standard (Structure of Management Information)
• At last, the object file defined in MIBs and located on the management station.

The SNMP protocol uses SNMP request and SNMP response packets called PDU - Protocol Data Unit.

Regarding the version of SNMP, orders are more or less different. In SNMP V1 there are five commands defined by RF 1157, SNMP v2 added two additional commands, disable one, added encryption and authentication.

SNMP packet uses list of OID (Object Identifier) to collect data. 

Packet types from manager

Get Request  : The SNMP Manager sends a request to a SNMP agent to collect one or more MIB variables on a specified objects.

Get Next Request : This request is made after the previous one only. The SNMP manager sends a request to a SNMP agent to collect one or more MIB variables on the next object in the hierarchy of the MIB. This command allows scan request on an indexed table object.

Get Bulk Request : Only in version 2 of SNMP, this request is set of a GET and GETNEXT and allows to obtain MIB variables on multiple object in one shot.

Set Request : Enable the SNMP manager to change one or more MIB variables. All MIB variables could not be set, some of them are read only.

Request types from agent

Get Response : Used by the agent to respond to a manager’s request.

Trap : The agent could send this unattended packet to inform the manager of a special an/or urgent event (state change of a link, failure ). The manager will not acquit this packet.

Notification : Introduce with version 2 of SNMP, the notification is sent by the agent to the manager to inform of a special an/or urgent event (state change of a link, failure ). The manager will not acquit this packet.

Inform : Introduce by version 2, the main goal of this packet is to allow manager to manage communication.

Here is an example of a GET on the object sysDescr and the corresponding response of the agent. The Name field contains the full Object Identifier OID resolved by the Lanalyzer software from Novell.

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