Step 2. The SPU for the Session Processes the Packet.

Most of a packet’s processing occurs on the SPU to which its session is assigned. The packet is processed for packet-based features such as stateless firewall filters, traffic shapers, and classifiers, if applicable. Configured flow-based security and related services such as firewall features, NAT, ALGs, and so forth, are applied to the packet. (For information on how security services are determined for a session, see Zones and Policies.)

1. Before it processes the packet, the SPU checks its session table to verify that the packet belongs to one of its sessions.
2. The SPU processes the packet for applicable features and services.

Example: SPU1 receives packet (a->b) from NPU1. It checks its session table to verify that the packet belongs to one of its sessions. Then it processes packet (a ->b) according to input filters and CoS features that apply to its input interface. The SPU applies the security features and services that are configured for the packet’s flow to it, based on its zone and policies. If any are configured, it applies output filters, traffic shapers and additional screens to the packet.

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