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EXPLAIN SNMP .
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It is an application program that allows 1. A manager to retrieve the value of an object defined in an agent 2. A manager to store a value in an object defined in an agent 3. An agent to send an alarm message about an abnormal situation to the manager

PDUs

SNMPv3 defines eight types of packets (or PDUs): GetRequest, GetNextRequest, GetBulkRequest, SetRequest, Response, Trap, InformRequest, and Report

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• GetRequest The GetRequest PDU is sent from the manager (client) to the agent (server) to retrieve the value of a variable or a set of variables.

• GetNextRequest The GetNextRequest PDU is sent from the manager to the agent to retrieve the value of a variable. The retrieved value is the value of the object following the defined Objectid in the PDD. It is mostly used to retrieve the values ofthe entries in a table. If the manager does not know the indexes of the entries, it cannot retrieve the values.

• However, it can use GetNextRequest and define the ObjectId of the table. Because the first entry has the ObjectId immediately after the ObjectId of the table, the value of the first entry is returned. The manager can use this ObjectId to get the value of the next one, and so on.

• GetBulkRequest The GetBulkRequest POD is sent from the manager to the agent to retrieve a large amount of data. It can be used instead of multiple

• GetRequest and GetNextRequest PODs. SetRequest The SetRequest PDD is sent from the manager to the agent to set (store) a value in a variable.

• Response The Response PDD is sent from an agent to a manager in response to GetRequest or GetNextRequest. It contains the value(s) of the variable(s) requested by the manager.

• Trap The Trap (also called SNMPv2 Trap to distinguish it from SNMPv1 Trap) POD is sent from the agent to the manager to report an event. For example, if the agent is rebooted, it infonns the manager and reports the time of rebooting. InformRequest

• The InfonnRequest POD is sent from one manager to another remote manager to get the value of some variables from agents under the control of the remote manager. The remote manager responds with a Response POD. Report The Report POD is designed to report some types of errors between managers. It is not yet in use.

• Format The fonnat for the eight SNMP PODs is shown in Figure The GetBulkRequest POD differs from the others in two areas, as shown in the figure.

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SNMP PDUformat

PDU type. This field defines the type of the POD

• Request ID. This field is a sequence number used by the manager in a Request POD and repeated by the agent in a response. It is used to match a request to a response.

• Error status. This is an integer that is used only in Response PDUs to show the types of errors reported by the agent. Its value is 0 in Request PDUs. Nonrepeaters. This field IS used only in GetBulkRequest and replaces the error status field, which is empty in Request PDUs.

• Error index. The error index is an offset that tells the manager which variable caused the errOr.

• Max~repetition. This field is also used only in GetBulkRequest and replaces the error index field, which is empty in Request PDUs.

• VarBind list. This is a set of variables with the corresponding values the manager wants to retrieve or set. The values are null in GetRequest and GetNextRequest. In a Trap PDU, it shows the variables and values related to a specific PDU.

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