Difference between CEF and Fast switching

[shah.chintan]

Hi,

Can any body tell me exact difference between CEF and fast switching.

Advantage of CEF over Fast switching ?

Advantage of Fast switching over CEF ?

With respect to Memory, CPU and other aspects.

Or any good document cisco to explain

Rgds

Chintan

[vijayasankar]

Hi Chintan,

Have a look at this URL..

http://www.cisco.com/warp/public/105/20.html

Hope this helps..

-VJ

[shah.chintan]

Thanks VJ . It explain well all diff switching technology.

But some more difference w.r.t. to CPU, Memory donesnt say anything. Can you give some more info on this

Rgds

Chintan

[vijayasankar]

Hi Chintan,

As far as i know and by the explanation provided in the above URL, CEF switching is better in terms of CPU and memory management.

AS you could see in that document, it explains the mechanism by which Fast switching works, which is surely consuming additional overhead for the CPU..(refer to the section Characteristics of Fast Switching). The router CPU has to spend a considerable amount of time, in building /managing and cleaning up (Aging Fast Switching Entries) its internal data structures to have a valid Fast switching cache entries.

Whereas when CEF is used, the whole process is more elegant.

Of Course the router has to maintain some data structure for managing CEF details, which in effect is going to consume some amount of memory. But what is more important here is, how efficiently the result is achieved.

As specified by Cisco, CEF is more efficient in terms on CPU and memory management.

-VJ

[ariazk]

check this out

http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/switch_c/xcprt2/xcdcef.htm

[Richard Burts]

One difference is that CEF builds its cache before packets are switched while fast switching is a demand cache that builds its cache based on the actual packets it has switched and builds the cache after it has process switched the first packet. The outcome of this is that fast switching must process switch at least the first packet to each destination. So there is an impact on CPU utilization for fast switching.

I believe that it is not significant but I believe that CEF may require a bit more memory since it is building cache entries for all possible destinations and since it builds the adjacency table which is not part of fast switching.

Another aspect is that fast switching periodically deletes some entries in the fast switching cache and refreshes the cache. This consumes CPU for fast switching which is not required in CEF.

As other responses have said in general CEF is a better technology than fast switching.

HTH

Rick

[carl_townshend]

how would i turn each of these on in my router ? and do they really make much difference ?