From: Scott Morris (smorris@ipexpert.com)
Date: Mon Mar 03 2008 - 03:19:26 ARST
I'm not entirely sure that helped the original poster's confusion either,
although potentially amusing to watch.
There's nothing wrong with Rik's post, except for missing the part about
that this is only the default behavior with CEF and can actually be changed.
Either way, the end premise of "CEF is usually a good thing" I'd agree with.
How picky you are becoming with your load-balancing will either increase
memory (up the CEF table and do per-packet lookups) or disable
CEF/fast-switching (up the CPU cycles), it's all a matter of choice.
In the lab, we generally don't care about the effects of either as we REALLY
don't have that much going on.
As a side note, if one is going to send provoking (or thought-provoking)
e-mails, it may assist to tag a link or some documentation in order to at
least diffuse the concept from an educational mishap versus an incite to
riot kinda thing. :)
http://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_referen
ce09186a00800afeb7.html
(Note, that's an old document, but it actually does a decent job of
explaining and showing things working!)
HTH,
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE-M
#153, JNCIS-ER, CISSP, et al.
CCSI/JNCI-M/JNCI-ER
VP - Technical Training - IPexpert, Inc.
IPexpert Sr. Technical Instructor
A Cisco Learning Partner - We Accept Learning Credits!
Telephone: +1.810.326.1444
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http://www.ipexpert.com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Tony
Varriale
Sent: Monday, March 03, 2008 12:00 AM
To: 'Rik Guyler'; 'groupstudy groupstudy'
Subject: RE: Load balancing
Any chance you would be able to do some research first and not propagate
incorrect information regarding CEF? It would certainly help the original
poster's confusion.
Tony
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Rik
Guyler
Sent: Sunday, March 02, 2008 10:31 PM
To: 'Gaurav Prakash'; 'groupstudy groupstudy'
Subject: RE: Load balancing
If you have CEF (Cisco Express Forwarding) enabled then you are running
per-destination load balancing. CEF relies on a route cache concept that
only performs a route table lookup on the first packet of the flow and then
relies on a hardware-based cache for the remainder of the packets. Since
the lookup only takes place once during the lifecycle of the flow the CPU
utilization is minimized and the actual packet switching process is much
faster. This is one of the fast switching paths in Cisco architecture.
In contrast, per packet load balancing, which is called process switching
(CEF/ip route cache disabled) requires a route table lookup for each packet
so the processor has to work extra hard to do this. The switching function
between interfaces is slower in this mode.
CEF is usually a good thing and typically turned on by default these days.
Many things require CEF, such as QoS but there are times when you need to
enable process switching (turn off CEF) such as when you want to run certain
packet debugs. In those cases process switching is the only solution as
fast switching will not take the packets through the debug process. You can
turn off fast switching globally by typing "no ip cef" or on an interface by
typing "no ip route-cache".
This is just a general explanation for typical devices. Some of the higher
end devices have more options than this.
Rik
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Gaurav Prakash
Sent: Sunday, March 02, 2008 11:01 PM
To: groupstudy groupstudy
Subject: Load balancing
Hi,
I have CEF on on my router, I want to be sure which method of load
balancing will eat more CPU.
Per packet Load balancing or Per Destination Load Balancing
A brief explanation plz..
Regards,
Gaurav
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