CEF-related terms

Answered Question
Apr 9th, 2009

Hi,

Can somebody explain to me the differences/similarities between these terms?:

L3 Engine, L3 Forwarding Engine, Control Plane, Forwarding Plane, Fast Switching

I have this problem too.
0 votes
Correct Answer by lamav about 7 years 9 months ago

OK, fair enough.

In the context of this book and this diagram, they are referring to the control plane as the L3 engine and the forwarding(data) plane as the L3 forwarding engine.

The L3 engine builds routing information that the L3 forwarding engine uses to forward data in hardware. <<--I am paraphrasing from the top of the drawing.

So, the L3 engine is the mechanism that builds the route table in software, in the control plane.

The L3 forwarding engine is the CEF table that was built from information taken from the route table and "downloaded" to an ASIC chip known as a TCAM. It operates in the data plane.

HTH

Please rate all helpful posts.

Thanks

Victor

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lamav Thu, 04/09/2009 - 04:09

L3 engine and L3 forwarding engine are the same thing. They are talking about routing. L3 means routing.

Control plane refers to a conceptual area where routers and L3 switches perform the information exchanges that inform their routing protocol databases, route tables and forward information databases. So, when two routers exchange routing information as they converge, they are said to be active on the control plane.

Forwarding plane is another term for data plane. It, too, is a conceptual area, but it refers to the area in which devices exchange user traffic between themselves.

For example, because of CEF and a feature known as NSF (non-stop forwarding), a switch supervisor in a L3 switch can be in the process of rebuilding its route table in the control plane, while using the pre-failure CEF table that was created to continue forwarding traffic on the data plane, ie forwarding plane.

Fast switching is a method by which Cisco routers cache active data flows between end users. So, instead of constantly doing a route lookup to find the next hop for a packets destination network, it will consult with the fast switching cache and look for the flow. The problem with this is that it can use up a lot of memeory and kill the router's CPU.

HTH

Victor

Wassim Aouadi Thu, 04/09/2009 - 07:11

Victor,

According to Cisco BCMSN Exam Guide 4th Ed. page 296, it says that L3 Engine and L3 Forwarding Engine are seperate components. Take a look at the diagram I sent below.

Any idea?

Attachment: 
Correct Answer
lamav Thu, 04/09/2009 - 07:18

OK, fair enough.

In the context of this book and this diagram, they are referring to the control plane as the L3 engine and the forwarding(data) plane as the L3 forwarding engine.

The L3 engine builds routing information that the L3 forwarding engine uses to forward data in hardware. <<--I am paraphrasing from the top of the drawing.

So, the L3 engine is the mechanism that builds the route table in software, in the control plane.

The L3 forwarding engine is the CEF table that was built from information taken from the route table and "downloaded" to an ASIC chip known as a TCAM. It operates in the data plane.

HTH

Please rate all helpful posts.

Thanks

Victor

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