NetFlow is a protocol that collects and exports information about network traffic flows. NetFlow Version 9 is the latest version supported by the Cisco ASA 5500 Series firewall. To enable NetFlow on the ASA, you need to perform two main tasks: define a NetFlow collector and apply NetFlow exporter to an interface. A NetFlow collector is a device or application that receives and processes the NetFlow records sent by the ASA. You can define a NetFlow collector by using the flow-export command, which specifies the IP address, port number, and interface of the collector. A NetFlow exporter is a configuration that determines which traffic flows are monitored and exported by the ASA. You can apply NetFlow exporter to an interface by using the Modular Policy Framework (MPF), which allows you to create class maps and policy maps to match and act on interesting traffic. You can use the flow-export event-type option in the policy map to select the NetFlow events that you want to export. The other options (B, C, and D) are not required or correct for enabling NetFlow on the ASA. You do not need to create an ACL to allow UDP traffic on port 9996, because the ASA uses a random high port number to send NetFlow records to the collector. You do not need to apply NetFlow exporter to the outside interface in the inbound direction, because you can apply it to any interface and direction that you want to monitor. You do not need to create a class map to match interesting traffic, because the ASA monitors all traffic flows by default, unless you filter them by using the flow-export filters command. References:
* Cisco Secure Firewall ASA NetFlow Implementation Guide, section "About NSEL"
* Cisco Secure Firewall ASA NetFlow Implementation Guide, section "Configure NSEL Collectors (CLI)"
* Cisco Secure Firewall ASA NetFlow Implementation Guide, section "Configure Flow-Export Actions Through Modular Policy Framework"
* Configuring NetFlow on ASA with ASDM, section "Enabling NetFlow on ASA"