Cisco ASA NetFlow v9 Secure Event Logging (NSEL) is a security logging mechanism that is built on NetFlow Version 9 technology and provides stateful, IP flow tracking for significant events in a flow. NSEL requires a collector to receive and process the exported data records. To configure NSEL, you need to define a flow-export event type under a policy using the Modular Policy Framework (MPF). The event type specifies which events to export, such as flow-create, flow-denied, flow-teardown, flow-update, or all. You also need to configure the NSEL collectors, the template timeout intervals, and the flow-update interval. Optionally, you can disable the redundant syslog messages and reset the runtime counters.
The other statements are false because:
To view bandwidth usage for NetFlow records, the QoS feature is not required. You can use a NetFlow collector or analyzer to view the bandwidth usage and other statistics from the exported data records.
A sysopt command cannot be used to enable NSEL on a specific interface. NSEL is enabled globally on all interfaces by default when you configure a flow-export event type under a policy.
NSEL cannot be used without a collector configured. NSEL exports data records to the configured collectors using NetFlow over UDP. If no collector is configured, the data records are discarded.
[References:, NetFlow Secure Event Logging (NSEL) - Cisco, Topic: Configuring NSEL, page 48-5, Cisco Secure Firewall ASA NetFlow Implementation Guide, Topic: Configure NSEL Collectors (CLI), Configure Flow-Export Actions Through Modular Policy Framework, Configure Template Timeout Intervals, Change the Time Interval for Sending Flow-Update Events to a Collector, Disable and Reenable NetFlow-related Syslog Messages, Reset Runtime Counters, Configuring NetFlow on ASA with ASDM - Cisco Community, Topic: Enable NetFlow (ASDM), , ]
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