Simple Network Management Protocol (SNMP)

A popular protocol for network management.

Overview

Simple Network Management Protocol (SNMP) is an Internet-standard
Layer-7 (application layer) protocol for collecting information from
and configuring network devices such as servers, printers, hubs,
switches, and routers on an Internet Protocol (IP) network. SNMP can
be used to collect information about network statistics from these
devices and to relay this information to a central management console
to monitor network health, trap errors, perform diagnostics, and
generate reports.  Typical statistical information might include the
number of packets or frames sent or received per second and the number
of errors per second. SNMP can also be used for reading and sometimes
modifying device configuration information such as the IP address of
an interface, or the version of the operating system running on the
device.  G0SXX07 (was G0Sui13.eps in 1E)

SNMP was developed in the late 1980s and is still the most popular
network management protocol in use.  There are two versions of SNMP:

. SNMPv1: This is the original version of SNMP, which lacked security
for ensuring the authenticity and integrity of SNMP messages.

. SNMPv2: This version has increased security that supports a simple
authentication scheme based on SNMP communities.  SNMPv2 is defined in
RFCs 1901 through 1908 and 2089.

Implementation

SNMP is a client/server protocol that consists of two components that
work together:

. SNMP agents: These are programs that run on the network devices to
be managed (called managed devices) and that collect Transmission
Control Protocol/Internet Protocol (TCP/IP).related configuration
information and statistics about the operation of the device. Agents
do not require heavy CPU usage to run. The types of information that
an agent can collect are defined in a local database called a
Management Information Base (MIB). MIB databases are hierarchical and
contain managed objects that have uniquely assigned identifiers issued
by the International Organization for Standardization (ISO). SNMP
variables are specific instances of managed objects in MIBs. Agents
running on managed devices monitor specific sets of SNMP variables and
temporarily store this information until the agent is polled by a
management system, whereupon the agent reports the values of the
stored information to the management system. Most network devices have
built-in SNMP agent software and associated MIBs.

. SNMP management system: Also called Network Management System (NMS),
this is software that runs on an administrative console and can
display data gathered from managed devices in a userfriendly form
through a graphical user interface (GUI). SNMP Management Systems
software can notify the administrator when certain conditions (such as
errors) occur. Most SNMP management systems can also automatically
determine the topology and components of a network with SNMPenabled
computers, routers, hubs, and switches and can display network
topology, traffic, and conditions in graphical format. SNMP management
systems regularly poll managed devices using SNMP messages for
statistical and configuration information and then store this
information in a central database, which can be used to present the
information in a friendly way to users.

SNMP is a simple protocol that is message-based in operation. SNMP
messages are sent between management consoles and managed devices over
User Datagram Protocol (UDP) port 161. These messages contain a header
and a payload called the Protocol Data Unit (PDU). The header contains
information about the community being referenced. A community is a
subset of agents that will be monitored using a specific management
system and institutes a primitive level of security, acting as a
primitive means of authentication. SNMP messages come in four types,
three of which are issued by management stations and one by agents:

. Get: Issued by the management system to an agent on a managed device
to read the value of a specific variable on the device.

. Getnext: Issued by the management system to determine which SNMP
variables are supported by an agent running on a managed device and to
traverse a series of variables to read their values sequentially.

. Set: Issued by the management system to an agent on a managed device
to write a value for a specific variable on the device.

. Trap: Issued by an agent running on a managed device when an error
or alert condition occurs. The trap message is sent to the management
system to alert administrators of the situation.  SNMP version 2 adds
additional security features, can be applied to network architectures
other than TCP/IP, and supports additional data types. It is only
partially backward compatible with SNMP 1. SNMP 2 also defines two
additional types of messages:

. Getbulk: Similar to getnext but allows the retrieval of greater
amounts of information in one data block

. Inform: Allows management systems to send information to other
management systems using a trap-like message

The management station regularly sends get, getnext, and set messages
to the SNMP agent on the managed device, in effect periodically
polling the agent for the status of the device. The agent verifies the
community name in the message, verifies the IP address or host name of
the SNMP management system, processes the request, and sends the
results to the management system.

Notes

A new version of SNMP called SNMPv3 is currently being developed to
enhance the security of earlier versions.