What is IP CEF?

I have a high memory & CPU loaded C2600 router & I noticed that when I
add (ip cef) to the configuration & the load decreases, so how this
command affecting that load on that router & does it have any side
effects on the performance of the router?

 

When a router receives a packet on a routed interface, it first
removes the Layer 2 (L2) frame information. It then stores the Layer 3
(L3) packet in Input/Output (I/O) memory. What happens next depends on
the switching path that the packet is following.

Cisco's Express Forwarding (CEF) is an advanced, Layer 3 switching
technology inside a router. It defines the fastest method by which a
Cisco router forwards packets from ingress to egress interfaces.

Process switching is the lowest common denominator in switching paths;
it is available on every version of IOS, on every platform.
I beleave this is the defaul method used.

Process switching uses the CPU on every packet, CEF only needs to the
CPU for the first packet of each session.

 

(hakim soso) wrote

Generally there are no adverse side effects from enabling CEF.

The main one to watch out for is that certain debugging will not work
for packets that are fast switched. I have spent many unhappy hours
struggling unsuccessfully to get the information that I wanted before
I realised what the problem was.

I have recently encountered a difference in behaviour between
fast switching and process switching where the firewall feature
was incorrectly blocking packets.


CEF is a "fast" switching method that allows the processing effort
required to "switch" a packet to be substantially reduced when
compared
to the traditional method of simply looking up the routing table for
each packet. The max forwarding rate of a router for any of the fast
switching methods is usually about 10 times the rate for process
switching. I would guess that CEF is the fast switching method to
use since it is the one that Cisco now most often recommend which
means that it will be the most often deployed and hopefully you will
be least likely to encounter a bug and cisco will be most likely
to be able to answer you support questions most effectively.

It has the advantage over the other methods of NOT needing to
process switch the first packet to each destination and can also
I suspect do per destination and per packet load balancing.


http://www.cisco.com/en/US/tech/tk827/tk831/tk102/tech_protocol_home.html
http://www.cisco.com/en/US/tech/tk827/tk831/technologies_white_paper09186a00800a62d9.shtml

When you configure the device in certain ways, fast switching can be
automatically disabled for some packets. For example policy routing or
fancy queuing may disable fast switching for the relevant traffic.
This was platform and software release dependant.