System information

390 Observer Suite: SNMP Management
When the management station needs information from an SNMP agent, it sends an SNMP
request. SNMP specifications allow the station to ask for more than one MIB object in a
single request.
When the SNMP agent receives the request, it searches its local MIB, finds the current
values of the requested data, forms a response packet (PDU), and sends the PDU back to
the management station.
The management station receives the PDU, decodes it from the SNMP PDU format, and
displays the information as a list or in a graphical format that allows the network manager
to view, analyze, and modify the information.
The following sections review the concepts above in more detail.
SNMP MIB Objects, Groups, and Addresses
A MIB is a set of SNMP objects organized in a tree address structure. Each object in a
MIB has a unique address called an object identifier, and each branch on the tree is
identified by a number. The ISO 8824 specification defines the lower branches of the
SNMP MIB tree as:
iso(1).org(3).dod(6).internet(1) or, as expressed in Structure of Management Information
(SMI) language, 1.3.6.1 (see illustration).
The SNMP tree resides under the Internet subtree. Four branches after the Internet subtree
can be used by SNMP: The directory(1) subtree is reserved for future use by OSI.
• The mgmt(2) subtree includes standard SNMP MIBs I or II (RFC1156 and
RFC1213).
• The experimental(3) subtree is reserved for Internet experiments.
• The private(4) subtree provides space for vendor-specific MIBs. All private MIBs are
located under enterprises(1) branch. Any private object ID (OID) should begin from
the base address 1.3.6.1.4.1.
The address 1.3.6.1.2.1 or iso.org.dod.internet.mgmt.mib represents the address of the
standard SNMP MIB I or II on the ISO tree. Inside the MIB branch, SNMP objects are
organized beneath higher level branches called MIB groups. Because of the large number
of objects—the standard MIB II includes almost two hundred—MIB groups have been
created to simplify addressing. Groups consist of related objects: for example, ICMP, TCP,
EGP, and other statistics object groups.
The object address is the path from the MIB's root to an object. For example, the object
sysDescr in the MIB(1) System Group has the address 1.3.6.1.2.1.1.1 (see illustration).
Types of SNMP MIB Objects
SNMP objects accommodate many different types of data in the tree structure, including
numbers, text, addresses, bitfield assigned descriptions, and object IDs. Two