Managing Your OSPF Network

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SNMP agents have two commands, GETRESPONSE and TRAP, which function as follows:

  GETRESPONSE. This command flows from the agent to the NMS in response to any of the following manager-based commands: GETREQUEST, GETNEXTREQUEST, or SETREQUEST. It is returned as a GETRESPONSE PDU (Protocol Data Unit) that contains several fields. These fields help to note whether the received command was processed and, if successfully received and processed, it returns a listing of occurrences of the managed objects polled and the current values for each of those managed objects.
  TRAP. This command is not generated as a response to a manager-generated command. Rather, it is an unsolicited response. This command is an alarm condition established by the manager. When a particular predefined activity occurs or a specific threshold is breached (that is, a link or router failure) the agent generates a Trap PDU to the manager so the necessary corrective actions can take place.

Managed Devices

A managed device is a network node that contains an SNMP agent and resides on a managed network. Managed devices collect and store management information and make this information available to network management systems (NMSs) using SNMP.

Managed devices, sometimes called network elements, can be routers and access servers, switches and bridges, hubs, computer hosts, or printers. Figure 9-4 shows the relationship between an NMS, Agents, and MIBs.

Figure 9-4  Network management model.

The Management Information Base

The MIB is an established database of the hardware settings, variables, memory tables or records stored within files. These records are called data elements.

Data elements contain the information concerning the status, configuration, and statistical information base used to define the functionality and operational capacity of each managed device. This information is referred to as a MIB (Management Information Base). Each data element is referred to as a managed object. These managed objects are comprised of a name, one or more attributes, and a set of operations that can be performed on the managed object.

MIBs and Object Identifiers

A MIB can be depicted as an abstract tree with an unnamed root. Individual data items make up the leaves of the tree. Object identifiers (OIDs) uniquely identify or name MIB objects in the tree. Object IDs are like telephone numbers; they are organized hierarchically with specific digits assigned by different organizations.

The object ID structure of an SNMP MIB defines three main branches: Consultative Committee for International Telegraph and Telephone (CCITT), International Organization for Standardization (ISO), and joint ISO/CCITT. Much of the current MIB activity occurs in the portion of the ISO branch defined by object identifier and dedicated to the Internet community.

The current Internet-standard MIB, MIB-II, is defined in RFC 1213 and contains 171 objects. These objects are grouped by protocol (including TCP, IP, User Datagram Protocol (UDP), SNMP, and others) and other categories, including “system” and “interfaces.”

The MIB tree is extensible by virtue of experimental and private branches. Vendors can define their own private branches to include instances of their own products. For example, Cisco’s private MIB is represented by the object identifier It includes objects such as HOSTCONFIGADDR, which is identified, by object ID (OID) The HOSTCONFIGADDR object specifies the address of the host that provided the host configuration file for a specific Cisco device.

The basic MIB structure, including MIB-II (object ID = and the Cisco private MIB (object ID =, is shown in Figure 9-5. A more detailed version of Cisco’s MIB is illustrated in Figure 9-6 later in the chapter.

Figure 9-5  Basic MIB Structure.

Figure 9-6  Network Management System requests flow chart.

Structure of Management Information (SMI) Definitions

The SMI specifies that all managed objects should have a name, a syntax, and an encoding. The name is the object ID, which was discussed in the preceding section. The syntax defines the object’s data type (for example, “integer” or “string”). Subsets of ASN.1 definitions are used for the SMI syntax. The encoding describes how the information associated with the managed object is formatted as a series of data items for transmission on the network. Another ISO specification called the Basic Encoding Rules (BERs), details SMI encodings.

SMI data types are divided into three categories: simple types, application-wide types, and simply constructed types.

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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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