ce. Choosing the right one-and knowing what to look for when there's a problem-will help the savvy ad- ministrator keep a network running at peak performance. A router must move packets from one interface to another, just like a switch. The decisions about how to move packets from one interface to another are based on the routing information base (RIB), which is built manually or by Layer-3 routing protocols. The RIB is essentially the routing table (see Chapter 9 for details). Nexus switches may have multiple RIBs called Virtual Route Forwarding (VRF) instances. See Chapter 18 for more details regarding VRFs. There are many types of switching within a Cisco router. They are divided into two categories: process switching and interrupt context switching. Process switching involves the processor calling a process that accesses the RIB, and waiting for the next scheduled execution of that process to run. Interrupt context 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 operation of your network, knowing 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 interfaces, all of which have access to in