Process Switched vs. Fast-Switched

aragoen_celtdra

While reading the BSCI self-study guide I came across some statements saying:

When a packet is process-switched, load balancing over equal-cost paths occurs on a per-packet basis.
When packets are fast-switched, load balancing over equal-cost paths is on a per-destination basis. (Teare & Paquet, p.101)

I'm not sure exactly what process-switching and fast-switching are, and the few resources I've googled didn't explain them to my satisfaction. I'm wondering if anyone here can help clarify these terms or point me towards a good reference. Thanks much.

scheistermeister

There are three types of switching (from slowest to fastest):

Processed switched
Fast-switched
Cef (Cisco express forwarding)

Processed switched relies on the processor to switch packets. With this every packet is inspected by the processor. Fast switching I don't remember off the top of my head but will look for where I read about it again real quick. Cef is all done in hardware (ASIC chip) and is the fastest form of switching.

Edit: Found it. Was in the MPLS section of ISCW. Been a while since I reviewed that part, no wonder I couldn't remember!

Cisco IOS Platform Switching Mechanisms
The Cisco IOS platform supports three IP switching mechanisms as listed in Figure :

* Process switching
* Fast switching
* Cisco Express Forwarding (CEF)

The original switching mechanism available on Cisco routers was process switching. However, process switching is very slow because it must find a destination in the routing table. This process can possibly result in a recursive lookup. Process switching must also construct a new Layer 2 frame header for every packet. As a result, process switching is no longer widely used in modern networks.

Cisco IOS platforms have the capability to overcome the slow performance of process switching. The platforms support several switching mechanisms that use a cache to store the most recently used destinations. A cache uses a faster searching mechanism than process switching does while storing the entire Layer 2 frame header to improve the encapsulation performance. In cache-driven switching, an entry is created in the cache when the first packet whose destination is not found in the fast-switching cache is process switched. The subsequent packets are switched in the interrupt code; this is how the cache improves performance.

The most recent and preferred Cisco IOS platform switching mechanism is CEF, which incorporates the best of the previous switching mechanisms. One of the benefits of CEF is that this mechanism supports per-packet load balancing, which was previously supported only by process switching. CEF also supports per-source or per-destination load balancing, fast destination lookup, and many other features that are not supported by other switching mechanisms.

CEF uses a Forwarding Information Base (FIB) to make IP destination prefix-based switching decisions. The FIB is conceptually similar to a routing table or information base. It maintains a mirror image of the forwarding information contained in the IP routing table. When routing or topology changes occur in the network, the IP routing table is updated, and those changes are reflected in the FIB. The FIB maintains next-hop address information based on the information in the IP routing table.

Because there is a one-to-one correlation between FIB entries and routing table entries, the FIB contains all known routes and eliminates the need for route cache maintenance that is associated with switching paths such as fast switching and optimum switching.

aragoen_celtdra

Awesome. Thanks man! So can I assume that fast switching and/or process switching are built in features of the router and not something that is turned on/off by configuration? The description is still a little vague for me. But it looks like it's something that will probably come to light as I read them over and over and come accross them in future readings.

I appreciate the effort of digging up the info though. If you can do me a favor, can you send me a link to where you found it (or if it's a book, what book it was)?

And if anyone else want to throw in tidbits of info I welcome some more knowledge sharing

scheistermeister

CEF is default. That bit I quoted was from the Cisco Net Acad, but I am sure it is the ISCW Self Study Guide.

tech-airman

aragoen_celtdra wrote:

Awesome. Thanks man! So can I assume that fast switching and/or process switching are built in features of the router and not something that is turned on/off by configuration? The description is still a little vague for me. But it looks like it's something that will probably come to light as I read them over and over and come accross them in future readings.

I appreciate the effort of digging up the info though. If you can do me a favor, can you send me a link to where you found it (or if it's a book, what book it was)?

And if anyone else want to throw in tidbits of info I welcome some more knowledge sharing

aragoen_celtdra,

Do you recall "store and forward" and "cut through" switching for switches?

kpjungle

aragoen_celtdra wrote:

Awesome. Thanks man! So can I assume that fast switching and/or process switching are built in features of the router and not something that is turned on/off by configuration? The description is still a little vague for me. But it looks like it's something that will probably come to light as I read them over and over and come accross them in future readings.

I appreciate the effort of digging up the info though. If you can do me a favor, can you send me a link to where you found it (or if it's a book, what book it was)?

And if anyone else want to throw in tidbits of info I welcome some more knowledge sharing

ip route-cache:

Using the route cache is often called fast switching. The route cache allows outgoing packets to be load-balanced on a per-destination basis rather than on a per-packet basis. The ip route-cache command with no additional keywords enables fast switching.

So using the "no ip route-cache", will make you process switch packets (per-packet).

Some more info here:

http://www.cisco.com/en/US/docs/ios/12_3/switch/command/reference/swi_i1.html#wp1110935