Cisco Systems RJ-45-to-AUX manual Switching Paths, Process Switching, Fast Switching

Switching Paths

The switch is commonly referred to in marketing terms as a Layer 2 device. If you keep thinking that way, this section will confuse you. By definition, switching paths are logical paths that Layer 3 packets follow when they are switched through a Layer 3 device such as a router or internal route processor. These switching types allow the device to push packets from the incoming interface to the interface where the packet must exit using switching paths or table lookups. By using switching paths, unnecessary table lookups can be avoided, and the processor can be freed to do other processing.

You’re probably wondering, “Sean, this is a switching book. Why am I learning about switching paths in Layer 3 devices?” Well, inside switches are Layer 3 devices such as the Route Switch Module (RSM), Multilayer Switching Module (MSM), Multilayer Switch Feature Card (MSFC), and NetFlow Feature Card (NFFC). Later in this book, I will cover trunk links, which are links that carry more than one VLAN. Doesn’t it seem logical that if you need to have a “router on a stick,” which is an external router used for interVLAN routing, it might help to know if the router you are using can handle the traffic for all of your VLANs? Better yet, you should learn the internal working paths and types of switching paths through the route processor. Let’s take a look at all the switching paths used on Layer 3 devices.

In this section, we will focus on the following switching path types and the functions of each:

∙Process switching

∙Fast switching

∙Autonomous switching

∙Silicon switching

∙Optimum switching

∙Distributed switching

∙NetFlow switching

Process Switching

Process switching uses the processor to determine the exit port for every packet. As a packet that needs to be forwarded arrives on an interface, it is copied to the router’s process buffer, where the router performs a lookup based on the Layer 3 destination address and calculates the Cyclic Redundancy Check (CRC). Subsequent packets bound for the same destination interfaces follow the same path as the first packet.

This type of switching can overload the processor. Making Layer 3 lookups the responsibility of the processor used to determine which interface the packet should exit takes away from more essential tasks the processor needs to handle. It is recommended that you use other types of switching whenever possible.

Fast Switching

Consider fast switching an enhancement to process switching. This switching type uses a fast switching cache found on the route processor board. The first received packet of a data flow or session is copied to the interface’s processor buffer. The packet is copied to the Cisco Extended Bus (CxBus) and then sent to the switch processor. If a silicon or autonomous switching cache does not contain an entry for the destination address, fast switching is used because no entries for the destination address are in any other more efficient caches.

Fast switching copies the header and then sends the packet to the route processor that contains the fast switching cache. If an entry exists in the cache, the packet is encapsulated for fast switching, sent back to the switch processor, and then buffered on the outgoing interface processor.

Note	Fast switching is used on the 2500 and the 4000 series of Cisco routers by
	default.

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