by Dave Stanford

Created on: April 27, 2010

The Simple Network Management Protocol (SNMP) is a protocol that is used to facilitate the exchange of management information between devices on a network. SNMP runs over UDP ports 161 and 162 and is found at the application layer of the OSI (Open System Interconnection) model. SNMP is used to monitor items such as network traffic, performance, and connectivity.

A network that runs SNMP is composed of three main elements:

1) Managed devices - These can be anything including routers, switches, servers, or hubs and snmp information is collected from these devices.

2) Agent - This is the SNMP software that runs on a managed device. It contains the MIB (Management Information Base) information which snmp reads and writes.

3) NMS (Network Management System) - This is the system that requests SNMP information and then receives and processes this information.

A management information base (MIB) is a key component of the SNMP architecture. The MIB contains the structure of all information on the device and is basically a file that contains a series of identifiers (OID's) that represent some information on this device. There are numerous MIB's supported on a network device, each representing a specific characteristic of the device. One of the most common MIB's seen is the IF-MIB. This is a file that contains all of the interface related statistics and data on a managed device.

When an NMS is looking to collect information from a managed device running an snmp agent it sends various types of requests to collect specific MIB information and the device will respond with information. There are five specific protocol data units (PDU) involved in this request for information and they include:

a) GetRequest - This is used to request and retrieve the value of a single OID. The OID must have an access value of read or read-write. An example of the syntax via a network management tool like NetSNMP would be:

snmpget -v 2c -c public 1.1.1.1 ifInOctets.1

- snmpget is the pdu

- -v specifies the snmp version which in this case is version 2c (other versions possible are v1 and v3)

- -c is the community string which is public in this case

- 1.1.1.1 is the IP address of the managed device running an snmp agent

- ifInOctets.1 is the specific OID

b) SetRequest - This is used to changed the value of a specific OID. The OID must have an access value of read-write. The command syntax would be:

snmpset -v 2c -c public 1.1.1.1 sysName.0 s "devicename"

- snmpset is the pdu

- s specifies a string

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