ip cef and ip packet debugs

Unanswered Question
May 5th, 2008

I was recently trying to do some debugs and did not get any output until I turned off CEF with "no ip cef". What are the ramifications of turning off CEF? I assume there are some performance issues but are there any IOS features that require CEF? I ask because I consider a performace hit while debugging as OK but if turning off CEF breaks other things then the debugging info recorded might not be reliable. I am most concerned with policy-routing, object tracking and so on.



I have this problem too.
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amritpatek Fri, 05/09/2008 - 08:13

Before using debugging ip packet, note that the router is doing fast-switching by default, or may be doing CEF switching if configured to do so. This means that, once those techniques are in place, the packet is not provided to the processor, hence the debugging does not show anything. For this to work, you need to disable fast-switching on the router with no ip route-cache (for unicast packets) or no ip mroute-cache (for multicast packets). This should be applied on the interfaces where the traffic is supposed to flow. Verify this with the show ip route command.

DIEGO ALONSO Mon, 05/12/2008 - 18:52

Thanks Jon. The doc certainly looks like it will help me with this.



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