type of switching method

Unanswered Question
Dec 9th, 2007

hi all,

from cisco docs i understood fast switching cache the first entry & second entry would be with shortcut and goes on till time out

i am confused in enabling switching method

we use to give ip route-cache flow command to monitor netflow statistics is that mean netflow switching (fast switcing) is enabled.

same command to enable netflow switching also

makes me confused.

I have this problem too.
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Richard Burts Mon, 12/10/2007 - 05:20


The situation about switching method is more simple now than it used to be. Recent versions of code implement CEF switching by default. CEF is similar to fast switching in that they both build a cache of entries used to speed up the packet forwarding decision. But where fast switching uses a demand cache which is built when the first packet is switched and will time out after some time, CEF builds its cache before the first packet is switched and therefore has better performance in packet switching than fast switching does (CEF does not need to process switch the first packet like fast switching does).

NetFlow is interesting when considering switching methods. While NetFlow was originally developed as a switching method (it built a cache entry for the flow which was used to switch all packets in the flow) Cisco pretty quickly dropped the switching method part of NetFlow and emphasized the netflow cache as a way to generate information about the traffic flowing through the router. So now running NetFlow has no impact on the switching method used by the router.

So in older versions of code there were multiple switching methods to consider and to configure, in recent code it is pretty much CEF switching.



shiva_ial Mon, 12/10/2007 - 05:38

hi richard,

thanks for your reply

still i am confused, if i put ip route-cache flow in interface to monitor netflow, will

router use netflow switching method or not

from previous post, you mean to say it will not, then how i am able to see cache information for that particular interface netflow enabled.

pls clarify.

Richard Burts Mon, 12/10/2007 - 05:52


If you put ip route-cache flow in interface config then the router will build the netflow cache and will generate statistics using that cache. But it does not use that cache to make packet forwarding decisions. The packet forwarding decision is made using CEF - at least that is the way that recent code works. If you go far enough back you could get to versions where enabling netflow would impact the switching method. But not in any recent code.

I hope that clarifies your question. If not ask again.



shiva_ial Mon, 12/10/2007 - 20:49

hi richard,

happy to see reply from u again,i started thinking of such things

since i am planning to do my ccie,so i thought of covering things clearly without confusion


my router details

cisco 2620XM

Version 12.3(18a)



1 fast ethernet & 2 serial interfaces

i have enabled netflow only in serial0/0,serial0/1 not used


router>sh interfaces switching


Throttle count 0

Drops RP 0 SP 0

SPD Flushes Fast 0 SSE 0

SPD Aggress Fast 0

SPD Priority Inputs 413582 Drops 0

Protocol IP

Switching path Pkts In Chars In Pkts Out Chars Out

Process 60834 13817578 237671 25485439

Cache misses 2911 - - -

Fast 17698810 3105510979 18807246 3143858411

Auton/SSE 0 0 0 0


Serial0/0 LINK CONNECTED TO delhi

Throttle count 0

Drops RP 0 SP 0

SPD Flushes Fast 0 SSE 0

SPD Aggress Fast 0

SPD Priority Inputs 374823 Drops 0

Protocol IP

Switching path Pkts In Chars In Pkts Out Chars Out

Process 3355679 733606093 3281909 357610922

Cache misses 0 - - -

Fast 18635288 2937914844 17612968 2891691687

Auton/SSE 0 0 0 0

Protocol Other

Switching path Pkts In Chars In Pkts Out Chars Out

Process 10 200 104951 1676776

Cache misses 0 - - -

Fast 207435 3318960 1 16

Auton/SSE 0 0 0 0

NOTE: all counts are cumulative and reset only after a reload.


i am able to see netflow traffic for the serial interface

now how to confirm what type of switching this router runs

whether process switching or netflow or it is running both based on inetrface basis

fyi: command to diasable netflow no ip route-cache command not given in my router

pls clarify



bvsnarayana03 Mon, 12/10/2007 - 21:44

Can you do "sh ip cef". If the output shows "%cef not running" then enable it.

If its enabled in the switch then output indicates prefix, next-hop etc. Then cef is the switching method used irrespective of what you have configured.

hope that clarifies.

pls rate all helpful posts.

shiva_ial Mon, 12/10/2007 - 21:57

hi narayanan,

cef is enabled on that router

my confusion is ,When this router is doing cef

how does the router gets cache information

without using fast switching method,which i am able to see in netflow collector

shiva_ial Mon, 12/10/2007 - 23:37

thanks once again for this valuable reply

narayanan & richard

i am very clear on this now


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