hi Priscilla,
I think CEF has an option of per-packet load sharing, which determines the
path on a per-packet base.
The real power comes from distributed CEF, where if you have a 75xx
or similar with VIP cards, it'll utilize the switching processors on VIP
cards to do
the switching. This is the main reason we use CEF I think, you'll be
surprised how
much the CPU util % drops.
Leon Chang
CCNP, CCDA
----- Original Message -----
From: "Priscilla Oppenheimer" <[EMAIL PROTECTED]>
To: "Adam Hickey" <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>
Sent: Saturday, December 16, 2000 12:15 PM
Subject: Re: Switching method help!!
> Sometimes I answer questions to try to bring out the experts, not because
I
> am an expert. &;-) But so far nobody has added to my comments about CEF,
so
> I will.
>
> From my additional reading on the topic, I think it's not quite right to
> say that one of the goals of CEF was to overcome the per-destination
> load-sharing disadvantage of fast-switching. It's more correct to say that
> CEF was designed for a different type of network. It solves a different
> problem.
>
> Fast switching speeds up forwarding in a typical small-to-medium network
> with lots of client-server traffic. With fast switching, a router learns
> the exit interface for a destination and caches this information, so that
> the next time traffic comes in for that destination, the router can check
> the cache and quickly forward the traffic without a routing-table lookup.
>
> CEF, on the other hand, speeds up forwarding in large networks with
dynamic
> traffic patterns, such as the Internet. CEF is useful for networks
> characterized by Web-based applications or interactive sessions, where
> there are numerous and ever-changing destinations.
>
> In networks with dynamic traffic patterns, fast switching cache entries
> change frequently. These changes can cause traffic to be process switched
> using the routing table, rather than fast switched using the route cache.
> With CEF, on the other hand, the Forwarding Information Base (FIB) lookup
> table contains all known routes that exist in the routing table. The FIB
> eliminates route cache turnover. CEF also offers full switching capacity
at
> each line card when distributed CEF (dCEF) mode is used.
>
> See this URL for more info:
>
>
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/swit
ch_c/xcprt2/xcdcef.htm
>
> Priscilla
>
> At 11:27 AM 12/15/00, Priscilla Oppenheimer wrote:
> >Fast switching definitely has the problem you refer to. Fast switching
> >offers per-destination load-sharing. Since the router caches information
on
> >how to forward traffic to a destination, all traffic to that destination
> >exits the same interface.
> >
> >One of the goals of CEF was to overcome this problem. I can't remember
the
> >details, but I think you can do more precise load-balancing with CEF.
I'll
> >see if I can dig up more details and get back to you. Or maybe some other
> >folks who use CEF can chime in.
> >
> >Priscilla
> >
> >At 12:31 PM 12/13/00, Adam Hickey wrote:
> > >Hey all,
> > >
> > >Question:
> > >1) On a router with multiple outbound paths, if fast switching (or CEF)
=
> > >is enabled and outbound traffic comes through towards a given =
> > >destination, is all traffic for that same destination then locked to
the =
> > >same path untill the route cache is cleared?
> > >
> > >2) If the above is yes, how long does the router maintain its route =
> > >cache?
> > >
> > >Purpose:
> > >We have a 7507 that will be running BGP over 2 T1's. We are attempting
=
> > >to establish load balancing both inbound and out. Halabi (2nd Ed.) in =
> > >Chapter 7 warns that to do this one must be familiar with the different
=
> > >methods of switching. Thus I am studying to find the pro's and cons of
=
> > >each. CEF or fast switching sounds about right, however, the above =
> > >question was raised as I was discussing this with our lead engineer.
> > >
> > >
> > >Thank You
> > >Adam
> > >[EMAIL PROTECTED]
> > >
> > >_________________________________
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> >
> >
> >________________________
> >
> >Priscilla Oppenheimer
> >http://www.priscilla.com
> >
> >_________________________________
> >FAQ, list archives, and subscription info:
> >http://www.groupstudy.com/list/cisco.html
> >Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
>
>
> ________________________
>
> Priscilla Oppenheimer
> http://www.priscilla.com
>
> _________________________________
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>
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