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CEF Vs Fast Switching

Hi, Can anyone tell me the diffrence between the two and any reference material which i can acess and go thru. I want to make a presentation on the same. Thanks in advance.


Re: CEF Vs Fast Switching


This doucment would make excellent material for your presentation:

Pls do remember to rate posts.


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Re: CEF Vs Fast Switching

These are the basic differences.

With Fast Switching, the forward information database (fib) is built gradually on an as needed basis. It starts empty and is built as users need to find destinations.

With the first packet a user sends to destination $x, the ip routing table (rib) is consulted to find the outbound interface toward the ip address of the destination, the arp table is consulted to find the mac addy of the next hop. Being that looking up the destination via the ip routing table/rib is done in software (process switched), it takes more resources to perform this action.

After the lookup, the destination ip and outbound interface is sent to the fib so all subsequent packets to this destination do not have to be looked up (per ip routing table/rib).

With CEF, the full RIB is pushed/mirrored to the CEF FIB after boot so the router/switch does not have to gradually build the CEF FIB. Also MAC addresses for the next hop are contained in the Adjacency table. There is a hash table with pointers from the FIB to the Adjacency Table. This makes it easier for this entire process of packet switching to be programmed into hardware. Subsequently, the first packet sent to a destination does not have to be sent to software unless particular security related features are enabled. In the case of 6500 switches with linecards containing DFC/Distributed Forwarding Cards, each linecard will have a copy of the CEF FIB and Adjacency Table so the “packet switching” can be done on the linecard.

Re: CEF Vs Fast Switching

Hi Donald,

I've readed ur last post, I would like to know one thing, As far as I know that MPLS is usinf (Fast Switching) Technology, and since the process of it's lookup is so different from CEF, Then Does CEF is better performance wise than using MPLS technology?

Thanks in Advance,

Community Member

Re: CEF Vs Fast Switching

Hi D,

Thank you for the explanation between Fast Switching and CEF. I have only one question related to CEF. You mention "paticular security related features" in the case of bumping packets to software. Would having in inbound ACL together with CBAC qualify as situation where packets are process switched?

Thanks in advance for any informaiton provided,


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