memory. 2. The interface processor generates a receive interrupt. During this time, the cen- tral processor determines the packet type (IP) and begins to switch the packet. 3. The processor searches the route cache for the following information: a. Is the destination reachable? b. What should the output interface be? Chapter 15: Switching Algorithms in Cisco Routers Routing Information Base IP_Input process (RIB) Processor Route cache Process memory Input Network Output interface Input/output memory interface Network media media processor processor Figure 15-3. Interrupt context switching C. What is the next hop? d. What should the MAC addresses be converted to? e. The processor then uses this information to rewrite the packet's MAC header. 4. The packet is copied to either the transmit or the output queue of the outbound interface. The receive interrupt is ended, and the originally running process continues. 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 retrieved from other sources. In the example shown in Figure 15-3, the source is called the route cache. As we'll see, each switching path has its own means of deter- mining, storing, and retrieving this information. The different methods are what separate the individual switching paths within the interrupt context switching group. Fast Switching Fast switching process-switches the first packet in a conversation, then stores information learned about the destination for that packet in a table called the route cache. Fast switching uses the binary tree format for recording and retrieving information in the route cache. Interrupt Context Switchi