Philosophically, there is a management function and a managed function. Their physical location in the network is not important. Managed elements may be physically collocated with the management function, or may be at the far edge of the network. The management function is typically located at the headquarters' location. However, it does not have to be that way. As a case in point, the major network providers, such as AT&T, Sprint, and WorldCom, have distributed Network Operations Centers (NOC).
The management function is the user's interface to the network management system, and it is typically a GUI, which provides status, reports, and statistics gathered from the managed function.
Figure 36-3 depicts the logical relationship between the managed and management functions. The managed element, or object, is the actual device, whether it is a hub, bridge, or router. The agent function is the program, or process, that runs on the managed system and provides the interface to the management function. Associated with the managed element or device is a set of attributes. These attributes may include memory size and utilization, interface speed, traffic load, and so on, depending on the type of device.
Graphical Unit Interface / (GUI)
Graphical Unit Interface / (GUI)
Figure 36-3: Logical relationship between the managed and manager functions As one might suspect, there is a protocol defined between the management function and the agent that defines the format and content of each of the commands, responses, and traps. RFC 1905 governs these protocols. RFC 1907, in turn, governs the content of the SNMP messages. SNMPv2, also known as MIB-II or mib-2, is the dominant definition set today and has superseded the SNMPv1 mib-1. Version 2 greatly expands the detail available from the managed agent.
The agent is the interface to the management function and may accept commands to set parameters (such as timeouts, thresholds, and, in some cases, system operational parameters, such as group membership addresses or routing tables) as well as provide status upon request. The agent can also send unsolicited messages (called traps) indicating alarm conditions. The name "trap" comes from the capability of the manager function to set a threshold (trap), and should this condition be met, the agent sends off an unsolicited message. All other messages sent by the managed function agent are a result of GetRequest or GetNextRequest queries.
It should be clear that the data reported by a device like a hub is a lot less complex than the data from something like a router. The hub needs to report Ethernet collisions on the LAN, the number of frames handled, the number of errored frames, and so on. The router, on the other hand, has routing tables, queue sizes, memory utilization, central processing unit (CPU) utilization, and so on.
To keep this all straight, the management function must know about the content of each MIB on each device. In other words, it must have a complete database identical to the one on the managed device so that when it receives the information, it can place it in the appropriate location. When a new device is installed in the network, its MIB content and format must be loaded into the machine (typically, a host or a workstation) providing the management function. Likewise, the agent software must be installed in the managed device.
This emphasizes an important point. Setting up and running a network management system is more than a casual undertaking. It requires a commitment on the part of management to provide the resources and purchase the appropriate tools.
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