From: Stefan Grey (examplebrain@hotmail.com)
Date: Tue Dec 26 2006 - 05:14:06 ART
1. Great explanation: So is do I correctly understand that: In fast
switching the route cache which it builds is the analog for FIB in CEF and
the analog of adjacency table is just the normal switching table??
2. Do I correctly understand that all procedures behind CEF does also the
CPU... it takes also some CPU resources but looking into FIB table takes
much less time than looking into routing table. Or CPU doesn't participate
at all in CEF??
Thanks.
>From: Guy Sherr <gsherr@gmail.com>
>To: Stefan Grey <examplebrain@hotmail.com>
>Subject: Re: CEF vs fast switching.
>Date: Mon, 25 Dec 2006 17:58:22 -0500
>
>Right on CEF and fast-switching. The process switching question, not
>already answered: process switching is what a router does when the packet
>has no apparent cache or cef entry. Without a near-line entry, the line
>driver copies the packet into RAM and interrupts the CPU to it. The CPU
>interrupt service routine examines the packet heading for a destination and
>actually searches for the best route in the routing table (that is, NOT the
>RIB, FIB, or any MPLS attachments; the routing table you can see with show
>ip route). So, process switching is CPU intensive, and it takes about 100
>times longer to actually forward the packet. One place I worked, a fault
>in the route architecture was causing a cascade of core routers to dump
>their fast-switch caches about every 45 minutes. Network throughput went
>from 200,000 frames per second to about 2,200 frames per second when the
>CPU was doing the process switching.
>
>regards,
>guy
>
>Stefan Grey wrote:
>>Guys could you please help me with understanding the following.
>>1) I want to understand how different switching methods function:
>>especially CEF, fastswitching, process Swithcing.
>>
>>I have read several articles about CEF... but still don't fully understand
>>how does it function. And what I don't understand most and want to
>>understand most... Why CEF is more efficient than fast switching and other
>>switching methods???
>>
>>1) Aboud CEF. Is my understanding correct as below???
>>As I understand CEF works as follows: It has FIB and adjacency table. Once
>>router or Layer 3 receives an IP packet it looks into FIB to determine the
>>next hop router. Next it looks in the adjacency table to match the
>>next-hop IP address with its MAC address and the output interface.. Than
>>it does the switching... Correct??? Please comment...
>>
>>2) I have read about Fast switching that it makes switching first based on
>>process switching and than it build some route cache... and than switches
>>based on this cache.... But I don't understand it completely... Could you
>>plese explain how it works and how does this route cache look like???
>>
>>
>>Well I would appreciate any explanations... and also links where to read
>>more detail about this.
>>Thank you very much.
>>
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