cef question

[jkay18041]

I was reading about CEF and how it can speed up traffic and reduce cpu usage. I have a cisco 1841 router and a 3750g switch. If I turn cef on in the router do I or should I also turn it on in the switch? Also on the switch it has a option for neighbor using layer 2 should I enable that as well? Any other suggestions on what I should turn on and what not on this?

Thank you

[Joseph W. Doherty · Accepted solution]

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Posting

Generally, CEF should be enabled on any L3 device that supports it.

Unclear what the 3750g L2 neighbor option is you're asking about.

Realize the 1841 was only designed for "slow" WAN links.  I.e. LAN link or "modern" high bandwidth WAN links will usually easily overload it.

For the 3750g, you'll want to insure you don't bust any of its TCAM limits, so you may need to change the SDM preference.  Also on the 3750g, it if has uplink ports, they may have more buffer resources than the copper edge ports.  (I.e. keep that in mind for infrastructure links or links to really busy hosts.)

[jkay18041]

If my connection is 10mb down and 5mb up that should be ok for the 1841 though?

Thank you for your help.

[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 would consider that borderline for a 1841.

The 1841 is rated at 75 Kpps for minimum size packets, but Cisco suggests to not run above about 75%, leaving us 56.25 Kpps.

15 Mbps (minimum size) require 22.32 Kpps, which is well under the targeted max of 56.25, but any process switching and/or some configuration features can use your CPU reserve rather quickly.

Assuming you don't saturate the link, all the time, and also assuming your traffic won't be all minimum sized packets, and lastly assuming your configuration isn't overly CPU hungry, you'll probably be okay.