Data forwarding is processed in both the upstream and downstream directions.
- Upstream process: The PIC (Physical Interface Card ) encapsulates packets into frames and sends them to the PFE. The processing module on the inbound interface parses the link-layer protocol information and identifies the packet type. Then, the upstream traffic classification module classifies traffic based on QoS configurations on the inbound interface. After traffic classification, the PFE searches for forwarding entries to determine where to forward the packets. For example, to forward IPv4 unicast packets, the PFE searches the FIB for the outbound interfaces and next hops of the packets based on the packets' destination IP addresses.
- Downstream process: The PFE encapsulates link-layer information in the packets based on the packet type and outbound interface type. For example, for an IPv4 packet destined for an Ethernet interface, the PFE obtains the MAC address of the next hop. Then the downstream traffic classification module classifies traffic based on the QoS configurations on the outbound interface. The outbound interface processing module encapsulates the packets with new Layer 2 headers and sends them to the PIC.