Chapter Five. Packet Forwarding

This chapter presents the process of packet forwarding in Cisco routers. The chapter provides information on IP input packet processing, IP packet forwarding, and IP packet delivery, including the details of normal destination-based routing, policy routing, access control lists, and other techniques used in Cisco IOS.

Overview of IP Forwarding

Let’s first go through a brief overview of the forwarding process to understand the path taken by transit IP packets inside a router. As in any generic router, packets processed by Cisco IOS go from the physical layer up to the network layer, where the forwarding decision is made, and then back to the physical layer of the output interface, as shown in Figure 5-1.

Figure 5-1. Routing ...

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