Specifications

SNMP Overview
March 28, 2011 Page 3 of 27
informationforaspecificvariable.Theagent,uponreceivingaGetorGetNextmessage,willissue
aGetResponsemessagetothemanagerwitheithertheinformationrequestedoranerror
indicationaboutwhytherequestcannotbeprocessed.
ASetmessageallowsthemanagertorequestachangeto
aspecificvariable.Theagentthen
respondswithaGetResponsemessageindicatingthechangehasbeenmadeoranerror
indicationaboutwhythechangecannotbe made.
Atraporinformmessageallowstheagenttospontaneouslyinformthemanagerofan
“important”eventinthenetwork.
TheSNMPmanager
andagentuseinformationintheMIBtoperformtheoperationsdescribedin
Table 1.
Trap Versus Inform Messages
Ascomparedtoearlierversions,SNMPv3providesahigherdegreeofreliabilityfornotifying
managementstationswhencriticaleventsoccur.Traditionally,SNMPagentscommunicated
eventstoSNMPmanagersvia“traps.”However,ifatemporarynetworkproblempreventedthe
managerfromreceivingthe trap,thenthetrapwouldbelost.SNMPv3provides
“informs”,which
areamorereliableformoftraps.TheSNMPagentinitiatestheinformprocessbysendingan
informrequesttothemanager.Themangerrespondstotheinformrequesttoacknowledge
receiptofthemessage.Iftheinformisnotreceivedbythemanager,theinformrequestwill
timeoutandanewinformrequestwillbesent.Subsequentinformrequestswillbesentas
previousrequeststime‐outuntileitheranacknowledgementisreceivedfromthemanager,or
untilapre‐specifiedretry‐countisreached.
Access to MIB Objects
SNMPusesthe f ollowingauthenticationmethodstograntuseraccesstoMIBobjectsand
functions.
Table 1 SNMP Message Functions
Operation Function
get-request Retrieves a value from a specific variable.
get-next-request Retrieves a value from a variable within a table.
1
1. With this operation, an SNMP manager does not need to know the exact variable name. A sequential search is
performed to find the needed variable from within a table.
get-bulk-request
2
2. The get-bulk operation is only supported in SNMPv2c or later.
Retrieves large blocks of data, such as multiple rows in a table, that would otherwise
require the transmission of many small blocks of data.
get-response Replies to a get-request, get-next-request, and set-request sent by a management
station.
set-request Stores a value in a specific variable.
trap | inform
3
3. Inform notifications are only supported in SNMPv3.
Unsolicited message sent by an SNMP agent to an SNMP manager when an event
has occurred.