SNMP Starter

SNMP is currently on v3.  v1, the original version, is the basis for SNMP and includes the main functions of SNMP including MIBs, Polling, and Trapping.  Technically there was a v2, but it is not used.  v2c is used instead of v2, and its main contributions were Informs, GetBulk and 64 bit counters.  v3 mainly added encryption and views.

Poll – A management station sends a request to the SNMP agent on the monitored device and requests one or more values be sent back to the management station.  If the request is valid, the monitored device internally formats the response and sends it back.  There are 4 types of requests:

Get – The SNMP Get command is used to retrieve one or more specifically requested values from a remote device.

GetNext – The SNMP GetNext command is used to retrieve the value of the next entry greater than the ID requested.    

GetBulk – The SNMP GetBulk command is designed for retrieval of larger number of values than the Get and GetNext commands support, including blocks of consecutive IDs.

Set – The Set command is used to remotely change or modify one or more values.  Changing the value may change the operation of the device.  

Trap – The SNMP agent can also send out unsolicited notifications to the management station when specific events occur.  The most basic form of these notifications is the Trap.  the SNMP agent sends a message to the management station with no idea if it got there or was understood once received.

Inform – The biggest limitation of the Trap is that there is no guarantee it was delivered to the management station, or that the management station knew what it was once it received it.  The Inform message was added in SNMP v2 to try to correct this problem.  The SNMP agent sends an Inform message to the management station and the management station then sends back a reply that it received the message.  If the SNMP agent does not receive a reply after a certain length of time, it assumes the message didn’t make it and sends the inform again.  The SNMP Agent continues to repeat this process until the remote end replies, or the max number of message attempts is reached.

One important note on Informs; a number of common monitoring/management stations do not support Informs and will not send back a reply.