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In the previous sections, we've vaguely referred to SNMP-capable devices and network management stations. Now it's time to describe what these two things really are. In the world of SNMP, there aretwo kind of entities: managers and agents . A manager is a server running some kind of software system that can handle management tasks for a network. Managers are often referred to as NetworkManagement Stations (NMSs).[*] An NMS is responsible for polling and receiving traps from agents in the network. A poll, in the context of network management, is the act of querying an agent (router, switch,Unix server, etc.) for some piece of information. This information can be used later to determine if some sort of catastrophic event has occurred. A trap is a way for the agent to tell the NMS thatsomething has happened. Traps are sent asynchronously, not in response to queries from the NMS. The NMS is further responsible for performing an actionbased upon the information it receives from theagent. For example, when your T1 circuit to the Internet goes down, your router can send a trap to your NMS. In turn, the NMS can take some action, perhaps paging you to let you know that something hashappened.
The second entity, the agent, is a piece of software that runs on the network devices you are managing. It can be a separate program (a daemon, in Unix language), or it can be incorporatedinto the operating system (for example, Cisco's IOS on a router, or the low-level operating system that controls a UPS). Today, most IP devices come with some kind of SNMP agent built in. The fact thatvendors are willing to implement agents in many of their products makes the system administrator's or network manager's job easier. The agent provides management information to the NMS by keepingtrack of various operational aspects of the device. For example, the agent on a router is able to keep track of the state of each of its interfaces: which ones are up, which ones are down, etc. The NMS...
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