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/switch_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]
> >
> >_________________________________
> >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
>
>_________________________________
>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
_________________________________
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
