CEF & ip cache

yuriz43

So I'm trying to figure out why I'm seeing entries in show ip cache, when I have CEF enabled globally, and on each individual interface on my router.

Per packet load-sharing is disabled
Fast switching type 1, interface type 18
IP CEF switching enabled
IP CEF Feature Fast switching turbo vector


According to cisco, when CEF switching is enabled, you should no longer see entries in show ip cache... I've done a clear ip cache, but a few entries still make their way into the table. My show ip cef table looks correct, so I am wondering if this is normal... Are some of my packets being punted down to the Fast Switching level??

kpjungle

yuriz43 wrote:

So I'm trying to figure out why I'm seeing entries in show ip cache, when I have CEF enabled globally, and on each individual interface on my router.

Per packet load-sharing is disabled
Fast switching type 1, interface type 18
IP CEF switching enabled
IP CEF Feature Fast switching turbo vector


According to cisco, when CEF switching is enabled, you should no longer see entries in show ip cache... I've done a clear ip cache, but a few entries still make their way into the table. My show ip cef table looks correct, so I am wondering if this is normal... Are some of my packets being punted down to the Fast Switching level??

This would be a guess from the hip. But not everything is switched by CEF, even though CEF is enabled, at least on switches which do the whole CEF thing in hardware, things like tunnel interfaces, NAT enabled interfaces and packets for the device itself. Maybe you can check it with the sh ip cef command, and see if it says "receive" in front of the things you see in the ip route cache. A follow up questions to this would be if router use CEF, and CEF is implemented in hardware on switches (FIB and Adjacency), then what is the purpose of using CEF on routers without ASIC hardware?

Just my two cents very early in the morning

yuriz43

I am running NAT/PAT on this router for my internal lan. I also have some access-lists setup... Could this cause Fast Switching to be used instead of CEF for some packets?

EDIT: After doing more testing I've discovered that when I turn off ip cef, my route cache grows extremely large, with many more entries. So obviously, when CEF is enabled it is working, but some packets are getting punted down. It would be interesting to know exactly why, but I have not been able to find a debug command that shines any light on what is happening.

kpjungle

yuriz43 wrote:

I am running NAT/PAT on this router for my internal lan. I also have some access-lists setup... Could this cause Fast Switching to be used instead of CEF for some packets?

EDIT: After doing more testing I've discovered that when I turn off ip cef, my route cache grows extremely large, with many more entries. So obviously, when CEF is enabled it is working, but some packets are getting punted down. It would be interesting to know exactly why, but I have not been able to find a debug command that shines any light on what is happening.

I havent tried it, but maybe the "debug ip cef receive" will give some more input as to whats going on?

yuriz43

I tried that.. It seems to show when packets get process switched. It does not show the difference between Fast Switched and CEF though..