ontext switching are shown in Figure 15-3. Interrupt context switching is a broad description that encompasses various switching paths-fast switching, optimum switching, and CEF-and includes the following steps: 1. The interface processor detects a packet and moves the packet into input/output memory. 2. The interface processor generates a receive interrupt. During this time, the central processor (CPU) determines the packet type (IP) and begins to switch the packet. 3. The processor searches the route cache for the following information and uses it to rewrite the packet's MAC header: Chapter 15: Switching Algorithms in Cisco Routers a. Is the destination reachable? b. What should the output interface be? C. What is the next hop? d. What should the MAC addresses be converted to? 4. The packet is copied to either the transmit queue or the output queue of the out- bound interface. The receive interrupt is ended, and the originally running process continues. 5. The output interface processor notices the packet in its queue and transfers the packet to the network media. The obvious difference is that there are only five steps, as opposed to seven for process switching. The big impact comes from the fact that the currently running process on the router is interrupted, as opposed to waiting for the next scheduled execution of the ip_input process. The RIB is also bypassed entirely in this model, and the necessary information is re- trieved from other sources. In the example shown in Figure 15-3, the source is called the route cache. As you'll see, each switching path has its own means of determining, storing, and retrieving this information. The different methods are what separate individual switching paths within the interrupt context switching group. Routing Information Base IP Input process (RIB) Processor Route cache Process memory Input Output Network interface media Input/output memory interface Network media processor processor Figure 15-3. Interrupt context switching Fast Switching Fast switching process-switch