Discussion

This chapter discusses how to configure the SNMP agent on the router with
SNMPv2 and SNMPv3 and illustrates some basic techniques for using SNMP to
query the router to collect information. Much of the information gathering done by
SNMP is done from the NMS system, either with GUI or CLI tools. Discussion of the
workings of the NMS systems is beyond the scope of this book; you should refer to
your NMS documentation. For more information about SNMP see SNMP, SNMPv2,
SNMPv3, and RMON 1 and 2 (Addison-Wesley).
SNMP Management Model
SNMP uses a client–server model. The SNMP client is called a manager, and the
server is called an agent. The managers are centralized systems on the network that
actively monitor the agents, which are the actual network devices, by querying and
collecting status and statistics information from them. Managers can run on PCs or
workstations but more often run on dedicated devices called NMS systems that are
developed and sold by third-party companies. An example is the HP OpenView Network
Node Manager product.
Agents are individual processes running on the network devices that are being managed.
These processes gather and store the status and statistics about their host platform
and send them to the managers primarily in one or two ways. When the agent
receives an SNMP Get request from the manager, as a Get, GetBulk, or GetNext
request, it responds with the requested information. The second way is that the
agent sends to the manager unsolicited notifications, called traps, that are triggered
by events on the agent. The SNMP manager can also modify information on the
agent by sending SNMP Set requests. A JUNOS router running SNMP is simply an
SNMP agent. There are two JUNOS SNMP processes (daemons in Unix terminology):
SNMPD, the SNMP process, and MIB2D, the MIB-II process. snmpd is the main
entry point, or master agent, for dealing with SNMP, and it communicates with
mib2d, which is a subagent.
MIBs and OIDs
SNMP agents store information in a Structure of Management Information (SMI),
which is a hierarchical database that is similar to the directory structure in a filesystem.
The individual files that store the information are called Management Information
Bases (MIBs). Each MIB contains nodes of information that are stored in a tree
structure. The tree contains branches, which move down from a root node. The
branches are similar to the directory names in a directory path. Each branch eventually
ends in a leaf, similar to a filename in a filesystem, that contains a specific piece
of information about the SNMP agent. Each branching point in the tree corresponds
to a MIBobject and is identified by a number and a text string. The series of numbers
that uniquely identifies a node or a leaf is called the Object Identifier (OID). As
examples, OID .1.3.6.1.2.1.1.4 corresponds to sysContact (system contact information)
in the standard MIB-II MIB, and OID .1.3.1.4.1.2636 corresponds to
130 | Chapter 4: SNMP
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