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64
SNMP O
VERVIEW
How an Agent
Works
Traps
An Agent facilitates communication with an SNMP Manager and the device or
application (managed resource ). The SNMP Manager sends an SNMP PDU to the
Agent. This PDU contains an encoded request (such as a request to GET the
value, or SET the value, of a specific managed object).
When the Agent receives the request, it parses the SNMP PDU (ASN.1 decoding)
to determine the type of request (for example, GET or SET) and the MIB group,
and then invokes the appropriate access function corresponding to the object
within the MIB group.
The access function retrieves the object's current value (for a GET request) or
modifies the object's value (for a SET request). The method used to perform the
GET or SET depends on where the managed object resides (for example, UNIX
kernel, shared memory, file, or in another process) and does not involve SNMP. If
the object resides in another process, you can use shared memory or a proprietary
protocol, such as Sun RPC/XDR or DCE RPC.
Depending on the value received from the access function, the SNMP Agent
returns one of the following responses to the agent core:
A value from a GET function
•
•
An OK from a SET function
An ERROR, if an error occurred
•
The Agent core receives the response from the Agent hook and builds the SNMP
PDU (ASN.1 encoding). The Agent then sends the response PDU to the SNMP
Manager.
In addition to retrieving data from the managed resource in response to requests
from a management station, Agents also send unsolicited messages to Managers
when a significant event is detected. This is called a trap notification. The fields
that comprise the SNMP trap PDU occur in the order shown below:
PDU Type
enterprise
The fields include:
PDU type identifies the packet as a trap notification.
•
Enterprise is the vendor identification (OID) for the network management
•
sub-system that generated the trap (.1.3.6.1.4.1.20212).
Agent address is the IP address of the node where the trap was generated.
•
•
Generic trap type is an integer in the range of 0 to 6. These values have the
meanings indicated in
STRM SNMP Agent
agent
generic
address
trap type
Table
A-5.
specific
time
trap type
stamp
variable
bindings
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