364 V7122 GatewayUser Guide Trap: A message generated asynchronously by network devices. It is an unsolicitedmessage from an agent to the manager.Each of these message types fulfills a particular requirement of network managers: Get request: Specific values can be fetched using the ‘get’ request to determine theperformance and state of the device. Typically, many different values and parameterscan be determined using SNMP without the overhead associated with logging into thedevice, or establishing a TCP connection with the device. Get Next request: Enables the SNMP standard network managers to ‘walk’ through allSNMP values of a device (using the ‘get-next’ request) to determine all names andvalues that an operant device supports. This is accomplished by beginning with the firstSNMP object to be fetched, fetching the next name with a ‘get-next’, and repeating thisoperation. Get-Bulk: Extends the functionality of Get-Next by allowing multiple values to bereturned for selected items in the request. This is accomplished by beginning with thefirst SNMP object to be fetched, fetching the next name with a "get-next", and repeatingthis operation. Set Request: The SNMP standard provides a method of effecting an action associatedwith a device (using the ‘set’ request) to accomplish activities such as disablinginterfaces, disconnecting users, clearing registers, and the like. This provides a way ofconfiguring and controlling network devices using SNMP. Trap message: The SNMP standard furnishes a mechanism by which devices can‘reach out’ to a Network Manager on their own (using a ‘trap’ message) to notify or alertthe manager of a problem with the device. This typically requires each device on thenetwork to be configured to issue SNMP traps to one or more network devices that areawaiting these traps.The above message types are all encoded into messages referred to as Protocol Data Units(PDUs) that are interchanged between SNMP devices.SNMP MIB ObjectsThe SNMP MIB is arranged in a tree-structured fashion, similar in many ways to a diskdirectory structure of files. The top level SNMP branch begins with the ISO ‘internet’directory, which contains four main branches: "mgmt" SNMP branch: Contains the standard SNMP objects usually supported (at leastin part) by all network devices. "private" SNMP branch: Contains those ‘extended’ SNMP objects defined by networkequipment vendors. "experimental" and "directory" SNMP branches: Also defined within the ‘internet’ rootdirectory, these branches are usually devoid of any meaningful data or objects.
