t switching involves the processor interrupting the current process to switch the packet. Interrupt context switching is further divided into three types: Fast switching Optimum switching Cisco Express Forwarding (CEF) Each method uses different techniques to determine the destination interface. Gener- ally speaking, the process of switching a packet involves the following steps: 1. Determine whether the packet's destination is reachable. 2. Determine the next hop to the destination, and to which interface the packet should be switched to get there. 3. Rewrite the MAC header on the packet SO it can reach its destination. While the information in this chapter may not benefit you in the day-to-day opera- tion of your network, understanding how the different choices work will help you understand why you should choose one path over another. Knowledge of your router's switching internals will also help you understand why your router behaves differently when you choose different switching paths. Figure 15-1 shows a simplified view of the inside of a router. There are three inter- faces, all of which have access to input/output memory. When a packet comes into an interface, the router must decide to which interface the packet should be sent. Once that decision is made, the packet's MAC headers are rewritten, and the packet is sent on its way. Input Output Network interface interface Network media media processor processor Input Output Network interface media Input/output memory interface Network media processor processor Input Output Network interface interface Network media media processor processor Packets must get from one interface to another. How the router decides which interface to switch the packet to is based on the switching path in use. Figure 15-1. Router switching requirements The routing table contains all the information necessary to determine the correct interface, but process switching must be used to retrieve data from the routing table, and this is inefficient. Interrupt context switching is typically