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.Switching is commonly defined as the process that takes charge of moving data units(frames or packets) through the anatomy of internetworking devices.From the time data 106 Chapter 4: Applying Cisco Troubleshooting Toolsunits arrive at an interface until they leave the router, several issues need to be constantlyaddressed.Where is the data unit stored? What type of information should accompany thedata unit? Where does the data unit go next? How is the next destination determined? Andwhich statistics need to be collected about this data unit? As you can imagine, the mere taskof moving a data unit (a packet, in the context of routers) from one place to the next is oneof the simplest and least resource-consuming internetworking tasks.Routing can be simply defined as the operation that attempts to select an output interfaceand perhaps a next hop for a packet based on the packet s destination address.Differentrouting processes perform the task of routing different protocols packets.For example, theIP routing process, which is enabled by default, handles routing of IP packets.The routingprocess makes its routing decision by consulting its routing table, which it builds andmaintains dynamically.The sources of information of a routing process for building itsrouting table are1 The network segments that the router is actively connected to.2 The usable static routes available in the router configuration.3 The dynamic routing entries that the routing protocols offer.4 The routing policies or restrictions that are imposed.5 The usable default routes available.Imagine that a router (call it R1) is receiving a bunch of packets from its ethernet 0 interfaceand all of these packets have the same destination address.When the router receives the firstlayer 3 packet (call it P1), the data link header (layer 2 frame) is discarded and the packetstored in the E0 interface s buffer.Next, the packet will be moved through R1 s internal businto main memory, then stored in a packet buffer where it will wait its turn to be examinedby the routing process.It is important to note that the packet is accompanied by additionalinformation such as the interface the packet entered the router from.When it is P1 s turn,the routing process uses the destination address of the packet and the routing table to selectan interface for the packet to leave the router from.Finally, a frame appropriate to thatinterface must be created.This entails building a frame header, usually composed of layer2 addressing and perhaps other information such as protocol type.Notice that during this process the packet is moved by the switching process from one placeto another (from the interface internal buffer to the main memory, for instance) severaltimes.Hence, the first packet (P1) must be handled by both the routing and the switchingprocesses.Performing both routing and switching tasks on a packet is called processswitching.Now think about the second packet (call it P2) that has the same destination asP1.Does P2 have to be routed and switched (i.e., process switched) as well? The answer isusually negative.To enhance efficiency and speed, the experience gained from the effort spent (through therouting task) on the first packet (P1, in our example) can be reused.P1 s destination addressand the outgoing interface (selected by the routing process) can be stored in a place referred Switching in 7000, 7500, 4000, 3000, and 2500 Series Routers 107to as a switching cache, and can be reused for quick processing of the subsequent packets(P2, P3, and so on, which have the same destination address as P1).Inside the switchingcache, the router needs to store the first packet s destination address along with the outputinterface number.The information stored in the switching cache is used to quickly discoverthe output interface for the following packets (destined for the same network as P1),without having to perform the time-consuming task of routing on each of them.You must remember that the router also caches information such as MAC addresses (in theARP cache) so that it does not have to generate an ARP request for every packet.Hence,when dealing with a number of packets (with identical destination addresses) arriving at therouter one after the other, the router must perform both the routing and the switching taskon the first packet.To process the following packets, using the information kept in the switching cache, therouter can skip the routing task.This is commonly referred to as route caching (and looselyspeaking, also called fast switching).Please keep in mind that there are more operations andtasks involved in the processing of each packet, most of which have to be performed foreach packet individually.For example, security tasks (checking access lists) andaccounting/queuing tasks still need to be performed for each packet (except in case ofNetflow switching, which is presented later in this chapter).Since the routing task is more resource consuming, is more complex, and introduces alonger latency, skipping this operation on all the packets except the first (all with the samedestination address) is very advantageous and efficient.When a packet (such as P2) is notprocess switched (not subjected to routing), the packet is said to be fast switched.Theswitch cache, where the network layer destination address and the corresponding selectedoutput interface (based on processing of the first packet) are stored, can be on one or moreof the router s components.Where that information is stored depends on the type of routerand its components, and based on that, the cache is referred to by a special term.Switching in 7000, 7500, 4000, 3000, and 2500 SeriesRoutersThis section discusses the switching options and their initialization on different routermodels.As you will notice, most of the methods are performing the same task, but someare faster than others due to highly specialized internal techniques and microchips
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