Is CEF required on access switches?

[gautamzone]

Dear friends,

There is a 4510-RE running with 12.2(53) code. We are using it as access switch.

During the POST of the switch, we get the following message:

% Enable CEF globally before configuring VRF on any interface

I am not sure if CEF is necessary on access switches which are not routing devices?

Can anyone share their thoughts / inputs on this?

Thanks a lot

Gautam                   

[KARUPPUCHAMY MALAIYANDI · Accepted solution]

Hi,

Cisco Express Forwarding (CEF) is advanced, Layer 3 IP switching technology. so there is no use of enabling n access-layer switches(L2).

Improved performance—CEF is less CPU-intensive than fast switching route caching. More CPU processing power can be dedicated to Layer 3 services such as quality of service (QoS) and encryption.

Scalability—CEF offers full switching capacity at each line card when distributed CEF (dCEF) mode is active.

Resilience—CEF offers unprecedented level of switching consistency and stability in large dynamic networks. In dynamic networks, fast switching cache entries are frequently invalidated due to routing changes. These changes can cause traffic to be process switched using the routing table, rather than fast switched using the route cache. Because the Forwarding Information Base (FIB) lookup table contains all known routes that exist in the routing table, it eliminates route cache maintenance and the fast switch/process switch forwarding scenario. CEF can switch traffic more efficiently than typical demand caching schemes.

Thanks & Regards

Karuppu

[Lei Tian · Accepted solution]

Hi Gautam,

In general a layer 2 switch doesnt require CEF. However if you need qos on the switch, you need to enable ip routing and cef. See the link's note.

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/53SG/configuration/qos.html

HTH,

Lei Tian

[Kevin Dorrell · Accepted solution]

Yes, I had noticed that I had to enable ip routing in order to make the QoS  work properly.  I never could understand why that should be so.  Can you shed any light on it please Lei?

Kevin Dorrell

Luxembourg

[francisco_1 · Accepted solution]

Kevin,

see this http://www.cisco.com/en/US/tech/tk39/tk824/technologies_tech_note09186a0080094978.shtml

[Kevin Dorrell]

Thank you Fransisco, that is a useful-looking document.

Kevin Dorrell

Luxembourg