Enabling CEF Question

[JohnTylerPearce]

Hey, would it be a wise move, to enable CEF on the WAN interface going to the Internet, if cef memory is currently at 98%.

AS of right now, this interface is process switching. The other two interfaces are cef switching.                  

[Richard Burts]

John

I would advocate for enabling CEF. You have not told us what is the platform or any other details. But I believe that you will be better off with CEF enabled.

HTH

Rick

[JohnTylerPearce]

Sorry about that Richard, it's a 2821 Router. This router has three gig interfaces, one going to the WAN (Internet Connection), the other one going to another switch.

It's currently set to 'no ip route-cache' so I can see it's process switch and would prefer to run the interface in CEF, but if CEF memory is currently at 98%, I don't want it to crash the router if I can help it

[Giuseppe Larosa · Accepted solution]

Hello John,

I agree with Rick.

Your router will work better with CEF enabled on all interfaces.

About memory usage by CEF: your device is a SW based router that already has built a CEF table to be used for CEF switching. The table is populated by routing information ( topology driven) so adding the third interface to CEF should not cause a big increase in memory usage as the CEF table is already built and you should be able to see lower CPU usage with all interfaces running CEF.

Hope to help

Giuseppe

[Joseph W. Doherty]

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Posting

John, I hope you realize CEF and interface "route-cache" are different features.

Like Rick and Giuseppe, I too would endorse using CEF, as Cisco does (for some newer IOS features, often CEF is a requirement).

I am confused a little by whether you're saying you're already using 98% of your memory for CEF or you're worried you might use that much..

[JohnTylerPearce]

Hey Joe, I know CEF and interface "route-cache" are different features. I may have done a typo I'm not sure. Our WAN interface to the internet has 'no ip route-cache' , which means it's process switching. When I do a 'show cef memory' and press the space bar a few times, it is always at 98%. So, I would love to turn on cef, but I'm afraid that it will crash the router. I know the IP_INPUT process is eating up quite a bit of processing power. This is a 2821 router.

[Joseph W. Doherty]

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

I've never used the show cef memory command, as generally I'm more interested in what show memory might show.  However, I just tried that command on a 6500, and it shows 99% total utilization.  This might relate to how much CEF is using of its allocated control structures.  I suspect it grows as needed (until you run out of total overall memory).

If you have sufficient RAM, it's probably just fine to enable CEF even though it shows 99% utilization.  (I would assume on your router it's at least globally enabled to get this stat at all.  You've disabled CEF on the outside facing interface?)

I recall (?) CEF is also fast path and overrides interface route-cache.  If I'm correct in what I recall, your process switching might have another cause.

[JohnTylerPearce]

Just enabled CEF switching on that interface, and can now get speeds of up to 95Mbps per WAN link. Process utilization went down almost 90%. Thanks for all the great help guys.

[Richard Burts]

John

Thanks for posting back to the forum and closing the loop on this discussion. It is good to know that our advice was on track and that CEF was helpful on your router. And it is especially good to have someone give the performance difference between CEF and not CEF based on real experience.

HTH

Rick