System information

SNMP Technical Overview 393
The GetRequest PDU is used by the management station to retrieve the values of one or
more objects from an agent. These values are usually singular, not columnar. When an
agent receives a GetRequest PDU, it checks the PDU for errors, finds the values
corresponding to the request packets, and sends a GetResponse PDU back to the
management station. If the error in the request packet occurs, the GetResponse PDU
returns an error message instead of the requested data. Errors can occur for the following
reasons:
• The variable bindings field does not exactly match the available object. In this case,
the GetResponse PDU returns a “noSuchName” error message.
• The variable is an aggregate type, such as a table object, in which case the return
message is “noSuchName.”
• The size of the GetResponse PDU would exceed the local protocol stack limitations.
In this case, the error message “tooBig” is returned.
The management station uses the GetNextRequest PDU to retrieve one or more objects
and their values from an agent. Usually these objects are multiple objects residing inside a
table. To retrieve all lines of the table, the management station starts at the beginning of a
table and sends GetNextRequest PDUs until all entries in the table are read. If no error
occurs, the agent returns the GetResponse PDUs on each of the GetNextRequest PDUs.
The SetRequest PDU is used by the management station to modify the value of an object
on the SNMP agent. If no error occurs, the agent sets a new value for the specified object
and returns a GetResponse PDU as a confirmation of the successful operation.
Agents send SNMP traps to the management station as notification regarding predefined
events. The trap PDU has a different format than the other four SNMP messages. On UDP,
traps are sent to port 160 on the management station. Because trap messages can be sent
from many different agents, the header of the trap PDU includes an enterprise OID and
agent address followed by the generic and specific trap types, timestamp, and the variable
bindings field.
There are seven generic trap types:
• “coldStart”—the SNMP agent device is reinitializing in a way that allows the device
or agent to be reconfigured.
• “warmStart”—the SNMP agent device is reinitializing in the way that does not allow
the device or agent to be reconfigured.
• “linkDown”—the SNMP agent detected a failure in the connection link.
• “linkUp”—the connection link came up.
• “authenticationFailure”—the SNMP management station did not properly
authenticate with the agent.
• “egpNeighborLoss”—an EGP peer of the SNMP agent is down.