In addition to consuming network bandwidth, polling routers for SNMP statistics also consume router CPU. Although Cisco routers prioritize all activities, and although SNMP operates generally at a low priority, it is still possible that you may adversely affect router performance by bombarding a router with an unreasonable level of SNMP requests. Moreover, because the router makes no distinction between SNMP requests, requests for superfluous SNMP data may override those for more critical data.
These are very conservative estimates; many integrated NMSs can easily—and often inadvertently, from a user's perspective—produce much greater levels of management traffic. Fortunately, assessing the overall level of management traffic is usually a relatively simple exercise of examining the input and output byte counts of your network management station.
Many other objects are available through various MIBs. In general, it is unnecessary to deploy polling of these on a large scale, unless they relate to a specific requirement of your network, such as whether they are critical for billing purposes or whether they significantly increase speed of fault isolation. One possible exception is for ISPs to poll for eBGP neighbor status using the BGP MIB. This is the most reliable way to detect that your neighbor ISP has deconfigured you as a BGP neighbor.
You may also decide to poll for certain objects that are precursors to faults. As an example, high ifInErrors may indicate that a linecard is beginning to fail; however, this is more in the realm of performance management, which is addressed later in this chapter.
Although a wealth of MIB objects exist for which you may not want to poll regularly, these can still be invaluable for "one-of" use in debugging specific problems. Such problems are typically more complex than your run-of-the-mill network device hardware failure: protocol, software, or configuration errors are examples.
SNMP server capability is enabled using the snmp-server community global configuration command. As you will discover when you read about configuration and security management, it is not recommended that you simply open SNMP access to just anyone.
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