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Network monitoring
Knowing what a running network does: counters that say how busy, flows that say who, logs that say what happened.
Three sources answer different questions. SNMP counters say how much crossed each card, polled every few minutes and turned into graphs. Flow records (NetFlow, IPFIX, sFlow) say who talked to whom, how much and when, without content. Logs (syslog) say what each device decided and why. A management network keeps all of it reachable when the production one is in trouble.