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SNMP Protocol Basic Request/Response Information Poll Using GetRequest and (Get)Response Messages
The obvious place to begin our detailed
look at SNMP protocol operations is with the simplest type of information
exchange. This would be a simple poll operation to read one or
more management information variables, used by one SNMP
entity (typically an SNMP manager) to
request or read information from another entity (normally an SNMP agent
on a managed device). SNMP implements this as a simple two-message request/response
protocol exchange, similar the request/reply processes found in so many
This information request process
typically begins with the user of an application wanting to check the
status of a device or look at information about it. As we've seen, all
this information is stored on the device in the form of MIB
objects. The communication, therefore,
takes the form of a request for particular MIB objects and a reply from
the device containing those objects' values. In simplified form, the
steps in the process are as follows (and as shown in Figure 274):
- SNMP Manager Creates GetRequest-PDU:
Based on the information required by the application and user, the SNMP
software on the network management station creates a GetRequest-PDU
message. It contains the names of the MIB objects whose values the application
wants to retrieve.
- SNMP Manager Sends GetRequest-PDU:
The SNMP manager sends the PDU to the device that is being polled.
- SNMP Agent Receives and Processes
GetRequest-PDU: The SNMP agent receives and processes the
request. It looks at the list of MIB object names contained in the message
and checks to see if they are valid (ones the agent actually implements).
It looks up the value of each variable that was correctly specified.
- SNMP Agent Creates Response-PDU:
The agent creates a Response-PDU to send back to the SNMP Manager.
This message contains the values of the MIB objects requested and/or
error codes to indicate any problems with the request, such as an invalid
- SNMP Agent Sends Response-PDU:
The agent sends the response back to the SNMP Manager.
- SNMP Manager Processes Response-PDU:
The manager processes the information in the Response-PDU received
from the agent.
Figure 274: SNMP Information Poll Process
The basic SNMP information polling process involves a simple exchange of a GetRequest-PDU sent by an SNMP manager and a Response-PDU returned by an SNMP agent.
Key Concept: The most basic type of communication in SNMP is an information poll, which allows a network management station to read one or more MIB objects from a managed node using a simple request/reply message exchange.
The Response-PDU message is
called GetResponse-PDU in SNMPv1. Presumably, this name was chosen
from the fact that it was a response to a get operation, to make the
names GetRequest-PDU and GetResponse-PDU somewhat symmetric.
The problem is that this name is confusing, for two reasons. First,
it sounds to some people like the purpose of the PDU is to get
a response. Second, the GetResponse-PDU was also defined
as the response message for operations other than gets,
including the reply message for SetRequest-PDU. Having a GetResponse
message be sent in reply to a SetRequest is disconcerting;
the new name is more generic and avoids these problems.
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Version 3.0 - Version Date: September 20, 2005
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