Managed network components

A typical managed network consists of the following components:

Managed nodes

An SNMP managed node can be any device that connects to a data network
and can execute the SNMP protocol. A managed node can be a:

• Host system, such as a workstation, printer, file
  server, terminal server, or mainframe.

• Network router, a bridge, a hub, an analyzer, or a
  repeater.

The following illustration shows managed nodes as grey and management
stations as white.

A managed node executes a program called the SNMP service, which communicates
with the management station. The SNMP service responds to messages from
the host and sends unsolicited messages if a defined event occurs on the
managed node.

The SNMP service is a daemon on UNIX systems and a system service under
Windows.

The SNMP service runs one or more agents that are applications that
collect information about the managed node and keep it in a MIB. A managed
node can have more than one MIB and has one agent for each MIB.

For information about how to activate the SNMP service and load an agent,
refer to the Installation
summary.

The SNMP architecture is designed to be simple and fast. The processing
load is placed on the management station and minimized on the managed
node. The set of information contained in the MIB is designed to be simple
so information about the network will not congest the network.

Management information bases

A management information base (MIB) defines the information maintained
by the associated agent. A MIB is viewed as a database, but is actually
a sequential list of managed objects. The managed objects are logically
grouped to represent a row in a table, where each object in that group
represents a field. The field can be a variable or a structure of variables.
Each managed object is assigned a unique object
identifier (OID).

A MIB is often shown as a tree, where the nodes of the tree define the
database and its tables, rows, and fields. The collection of all MIBs
is organized in a tree structure, where each node on the tree represents
a single MIB. The SNMP MIB hierarchy is defined by RFC 1155 and RFC 1213.
MIBs fall into two categories:

MIB

Description

Standard

A standard MIB is defined by the IETF. An
example of a standard MIB is RFC 2495, the Trunk MIB.

Private

A non-standard, proprietary MIB is defined
by an enterprise. The IETF assigns a unique OID number to a company,
under which they can define their own OIDs for their specific
products.

Management stations

A management station is a system running:

• The network management protocol.

• One or more network management applications.

The network management station (host) determines the information required
from the managed node. The host sends queries to a managed node to determine
what information is available and to retrieve that information. The host
then uses those responses to display the information in a readable format.

Host applications are larger than agent applications because they are
designed to do most of the work in the SNMP architecture and because one
host application communicates with many agents. An example of a host management
station is HP Openview.

Management protocol

SNMP defines a mechanism to transport network management information.
Messages containing queries and replies are sent between the host management
system and managed nodes over a connectionless transport service. A commonly
used transport service is user datagram protocol (UDP), which is part
of the IP suite.

SNMP supports the following message types:

Message

Description

Traversal

Provides a way for the host to read the values
in an agent’s MIB.

Trap

Notifies the host of events received by the
agent.

The following illustration shows the SNMP host and SNMP agent message
flow:

The following host commands generate traversal messages:

Command

Description

get

Requests
a specific value (for example, the amount of hard disk space available).

get-next

Requests the next value in a MIB after using
the get command. Useful when getting a block of related objects.

set

Changes the value of an object in a MIB.
Only objects with read-write access can be set.

An agent sends trap messages to notify the host about an unusual occurrence.
The host can then request the value of related variables to determine
more about the managed node’s condition. The agent can be set to send
a trap when certain conditions arise, such as an error on a line. Care
must be taken to ensure that trap information does not congest the network
or overwhelm the host.

Connectionless transport does not guarantee delivery. Traps and other
network messages are not guaranteed to arrive at the host. Plan your network
management policies to consider lost messages.