SNMPv2 defines the get-bulk operation, which allows a management application to retrieve a large section of a table at once. The standard get operation can attempt to retrieve
more than one MIB object at once, but message sizes are limited by the agent's capabilities. If the agent can't return all the requested responses, it returns an error message with no data. The get-bulk operation, on the other hand, tells the agent to send
as much of the response back as it can. This means that incomplete responses are possible. Two fields must be set when issuing a get-bulk command: nonrepeaters and max-repetitions. Nonrepeaters tells the get-bulk command that the first N objects can be retrieved
with a simple get-next operation. Max-repetitions tells the get-bulk command to attempt up to Mget-next operations to retrieve the remaining objects.
Assume we're requesting three bindings: sysDescr, ifInOctets, and ifOutOctets. The total number of variable bindings that we've requested is given by the formula N + (M
* R), where N is the number of nonrepeaters (i.e., scalar objects in the request -- in this case 1, because sysDescr is the only scalar object), M is max-repetitions (in this case, we've set it arbitrarily to 3), and R is the number of nonscalar objects in
the request (in this case 2, because ifInOctets and ifOutOctets are both nonscalar). Plugging in the numbers from this example, we get 1 + (3 * 2) = 7, which is the total number of variable bindings that can be returned by this get-bulk request.
The Net-SNMP package comes with a command for issuing get-bulk queries. If we execute this command using all the parameters previously discussed, it will look like the
following:
$ snmpbulkget -v2c -B 1 3 linux.ora.com public sysDescr ifInOctets ifOutOctets
system.sysDescr.0 = "Linux linux 2.2.5-15 #3 Thu May 27 19:33:18 EDT 1999 i686"
interfaces.ifTable.ifEntry.ifInOctets.1 = 70840
interfaces.ifTable.ifEntry.ifOutOctets.1 = 70840
interfaces.ifTable.ifEntry.ifInOctets.2 = 143548020
interfaces.ifTable.ifEntry.ifOutOctets.2 = 111725152
interfaces.ifTable.ifEntry.ifInOctets.3 = 0
interfaces.ifTable.ifEntry.ifOutOctets.3 = 0
Since get-bulk is an SNMPv2 command, you have to tell snmpgetbulk to use an SNMPv2 PDU with the -v2c option. The nonrepeaters and max-repetitions are set with the -B 1
3 option. This sets nonrepeaters to 1 and max-repetitions to 3. Notice that the command returned seven variable bindings: one for sysDescr and three each for ifInOctets and ifOutOctets.
Non-repeaters and maxRepetitions
They are used in getBulk.
Definition of Non-repeaters:- The Non-repeater specifies the number of variables in the variable-bindings list for which a single OID (lexicographic successor) is to be
returned.
Definition of maxRepetitions :- The max-repetitions specifies the number of OIDs (lexicographic successor)to be returned for the remaining variables (total variables -
nonrepeaters)in the variable bindings list.
For clearer understanding, Let us assume Nonrepeater=4, and Max-Repetitions=3;
If get values with OID lists which are .1.3.6.1.2.1.11.1.0, .1.3.6.1.2.1.11.2.0 , .1.3.6.1.2.1.11.3.0, .1.3.6.1.2.1.11.4.0, .1.3.6.1.2.1.11.5.0, .1.3.6.1.2.1.11.6.0 , and
the method is getNext.
NonRepeater value is 4. So the first four variable returns a single lexicographic successor.
Request OIDs ----> Response
.1.3.6.1.2.1.11.1.0 ---> .1.3.6.1.2.1.11.2.0 and its value
.1.3.6.1.2.1.11.2.0 ---> .1.3.6.1.2.1.11.3.0 and its value
1.3.6.1.2.1.11.3.0 ---> .1.3.6.1.2.1.11.4.0 and its value
.1.3.6.1.2.1.11.4.0 ---> .1.3.6.1.2.1.11.5.0 and its value
The subsequent OIDs in the OIDs list to be returned the number of max-repetitions lexicographic successor.
Request ---> Response
.1.3.6.1.2.1.11.5.0 --> .1.3.6.1.2.1.11.6.0, .1.3.6.1.2.1.11.7.0, .1.3.6.1.2.1.11.8.0 and its value.
Request ---> Response
.1.3.6.1.2.1.11.6.0 --> .1.3.6.1.2.1.11.7.0, .1.3.6.1.2.1.11.8.0 , .1.3.6.1.2.1.11.9.0 and its value.
So the response will be,
.1.3.6.1.2.1.11.2.0 and its value
.1.3.6.1.2.1.11.3.0 and its value
.1.3.6.1.2.1.11.4.0 and its value
.1.3.6.1.2.1.11.5.0 and its value
.1.3.6.1.2.1.11.6.0 and its value
.1.3.6.1.2.1.11.7.0 and its value
.1.3.6.1.2.1.11.7.0 and its value
.1.3.6.1.2.1.11.8.0 and its value
.1.3.6.1.2.1.11.8.0 and its value
.1.3.6.1.2.1.11.9.0 and its value