At the beginning of this chapter we took a look at the process by which a router will accept a frame on one interface, strip away its framing, determine where it should be sent next according to its routing table, reframe the packet, and ultimately forward the frame to the next hop or destination host. The manner is which this process occurs differs based on the switching path method employed by the router. In this case, “switching path” refers to the manner in which a router will accept a packet on one interface, process it, and ultimately forward it out another interface. There are 8 main switching path methods used on Cisco routers that you should be familiar with, as listed below. Note that not all methods are supported on all routers – in fact, some of the faster methods are available only on very high-end models.
Process Switching. With this switching method, incoming packets are copied to the router’s buffers, associated with a destination network according to a routing table entry, encapsulated, and then forwarded out the appropriate interface. The router’s CPU processes every packet in process switching.
Fast Switching. Fast switching handles the first packet in a stream just like process switching, but then creates a fast switching cache against which following packets are compared. Subsequent packets in the same stream have their incoming frame header stripped off and compared to the first packet. When a match is found, the header appended to the first frame is appended to subsequent frames prior to forwarding. This method helps to eliminate the need for routing table lookups for each packet in the same stream, increasing router throughput. Fast switching is the default method on lower-end Cisco routers.
Optimum Switching. Though it works in a manner similar to fast switching, optimum switching is faster due to the optimized cache lookup process that it uses. Optimum switching is the default method use on Cisco 7500 series routers.
Silicon Switching. This method uses a dedicated processor known as a silicon switch processor (SSP) module to cache packet switching information. This method allows the switching process to take place without interrupting the router CPU. Silicon switching is available on Cisco 7000 series routers only.
Autonomous Switching. Another very fast switching method found on Cisco 7000 series routers, autonomous switching allows the ciscoBus (cBus) controller to switch packets without the need for CPU intervention.
Distributed Switching. This switching method allows the switching function to take place locally via a route cache stored on a Versatile Interface Processor (VIP) card. This eliminates the need to use the router CPU to perform switching functions.
NetFlow Switching. This switching method allows you to collect detailed statistics on the traffic switched through the router for the purpose of accounting, planning, and network management. Because of the overhead associated with gathering this data, NetFlow switching is generally the slowest switching method.
Cisco Express Forwarding (CEF). An increasingly popular switching method aimed primarily at high-performance IP backbone switching. Less CPU-intensive than the fast switching method looked at first, CEF uses two main components to make switching decisions – a forwarding information base (FIB) and adjacency table. The FIB contains next-hop information for all IP networks in the routing table, and the adjacency table stores information on associated Layer 2 addresses. With this information, CEF is capable of switches packets faster than through the use of some of the multilayer shortcut switching methods looked at early in this chapter. CEF is no longer limited to Cisco routers; it is also now commonly found in Catalyst multilayer switches.