Simple Network Management Protocol (SNMP).pdf

CHAPTER

Chapter Goals

• Discuss the SNMP Management Information Base.

• Describe SNMP version 1.

• Describe SNMP version 2.

Simple Network Management Protocol

Background

The Simple Network Management Protocol (SNMP) is an application layer protocol that facilitates the

exchange of management information between network devices. It is part of the Transmission Control

Protocol/Internet Protocol (TCP/IP) protocol suite. SNMP enables network administrators to manage

network performance, find and solve network problems, and plan for network growth.

Two versions of SNMP exist: SNMP version 1 (SNMPv1) and SNMP version 2 (SNMPv2). Both

versions have a number of features in common, but SNMPv2 offers enhancements, such as additional

protocol operations. Standardization of yet another version of SNMP—SNMP Version 3 (SNMPv3)—is

pending. This chapter provides descriptions of the SNMPv1 and SNMPv2 protocol operations. Figure

56-1 illustrates a basic network managed by SNMP.

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Chapter 56

Simple Network Management Protocol

SNMP Basic Components

Figure56-1 SNMP Facilitates the Exchange of Network Information Between Devices

SNMP Basic Components

An SNMP-managed network consists of three key components: managed devices, agents, and

network-management systems (NMSs).

A managed device is a network node that contains an SNMP agent and that resides on a managed

network. Managed devices collect and store management information and make this information

available to NMSs using SNMP. Managed devices, sometimes called network elements, can be routers

and access servers, switches and bridges, hubs, computer hosts, or printers.

An agent is a network-management software module that resides in a managed device. An agent has

local knowledge of management information and translates that information into a form compatible with

SNMP.

An NMS executes applications that monitor and control managed devices. NMSs provide the bulk of the

processing and memory resources required for network management. One or more NMSs must exist on

any managed network.

Figure 56-2 illustrates the relationships of these three components.

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Chapter 56 Simple Network Management Protocol

SNMP Basic Commands

Figure56-2 An SNMP-Managed Network Consists of Managed Devices, Agents, and NMSs

Management

entity

NMS

Agent

Management

database

Management

database

Management

database

Managed devices

SNMP Basic Commands

Managed devices are monitored and controlled using four basic SNMP commands: read , write , trap ,

and traversal operations.

The read command is used by an NMS to monitor managed devices. The NMS examines different

variables that are maintained by managed devices.

The write command is used by an NMS to control managed devices. The NMS changes the values of

variables stored within managed devices.

The trap command is used by managed devices to asynchronously report events to the NMS. When

certain types of events occur, a managed device sends a trap to the NMS.

Traversal operations are used by the NMS to determine which variables a managed device supports and

to sequentially gather information in variable tables, such as a routing table.

SNMP Management Information Base

A Management Information Base (MIB) is a collection of information that is organized hierarchically.

MIBs are accessed using a network-management protocol such as SNMP. They are comprised of

managed objects and are identified by object identifiers.

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Chapter 56

Simple Network Management Protocol

SNMP Management Information Base

A managed object (sometimes called a MIB object, an object, or a MIB) is one of any number of specific

characteristics of a managed device. Managed objects are comprised of one or more object instances,

which are essentially variables.

Two types of managed objects exist: scalar and tabular. Scalar objects define a single object instance.

Tabular objects define multiple related object instances that are grouped in MIB tables.

An example of a managed object is atInput, which is a scalar object that contains a single object instance,

the integer value that indicates the total number of input AppleTalk packets on a router interface.

An object identifier (or object ID) uniquely identifies a managed object in the MIB hierarchy. The MIB

hierarchy can be depicted as a tree with a nameless root, the levels of which are assigned by different

organizations. Figure 56-3 illustrates the MIB tree.

The top-level MIB object IDs belong to different standards organizations, while lower-level object IDs

are allocated by associated organizations.

Vendors can define private branches that include managed objects for their own products. MIBs that have

not been standardized typically are positioned in the experimental branch.

The managed object atInput can be uniquely identified either by the object

name—iso.identified-organization.dod.internet.private.enterprise.cisco.temporary

variables.AppleTalk.atInput—or by the equivalent object descriptor, 1.3.6.1.4.1.9.3.3.1.

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Chapter 56 Simple Network Management Protocol

SNMP and Data Representation

Figure56-3 The MIB Tree Illustrates the Various Hierarchies Assigned by Different Organizations

ccitt (0)

iso (1)

iso-ccitt (2)

standard (0)

registration-

authority (1)

member-

body (2)

identified-

organization (3)

dod (6)

internet (1)

directory (1)

mgmt (2)

experimental (3)

private (4)

security (5)

snmpV2 (6)

mib-2 (1)

enterprise (1)

cisco (9)

temporary

variables (3)

DECnet (1)

XNS (2)

Apple Talk (3)

Novell (3)

VINES (4)

Chassis (5)

atInput (1)

atLocal (2)

atBcastin (3)

atForward (4)

SNMP and Data Representation

SNMP must account for and adjust to incompatibilities between managed devices. Different computers

use different data representation techniques, which can compromise the capability of SNMP to exchange

information between managed devices. SNMP uses a subset of Abstract Syntax Notation One (ASN.1)

to accommodate communication between diverse systems.

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