North-South network introspection is the process of inspecting traffic flowing between the NSX environment and the external network. This can be done using a variety of security tools, such as firewalls, intrusion detection systems, and data loss prevention systems.
The Tier-0 gateway is the ideal place to insert North-South network introspection tools because it is the single point of entry and exit for all traffic between the NSX environment and the external network. This allows the security tools to inspect all traffic, regardless of which Tier-1 gateway or segment the traffic is flowing to or from.