Understanding MSFC, CEF and VRF

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Feb 10th, 2010
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Hi,


I am looking for a a very simple explanation of the following terms


1. CEF

2. MSFC

3. TCAM

4. VRF


I know what they do and all that but I am not sure of the concept behind these technologies. I have read loads of documents on CEF and MSFC and all that but most are configs which I can do with ease without really understanding the technologies.


Any links pointing to a theoritical concept behind them would be very appreciative.


Thanks

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Jon Marshall Wed, 02/10/2010 - 09:58
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sidcracker wrote:


Hi,


I am looking for a a very simple explanation of the following terms


1. CEF

2. MSFC

3. TCAM

4. VRF


I know what they do and all that but I am not sure of the concept behind these technologies. I have read loads of documents on CEF and MSFC and all that but most are configs which I can do with ease without really understanding the technologies.


Any links pointing to a theoritical concept behind them would be very appreciative.


Thanks


1) CEF (Cisco Express Forwarding) is just one way in which Cisco device can forward packets. Others are process switched and fast switched. CEF is the most efficient and is the one in use on majority of Cisco devices. Because you have read lots of docs it's unclear exactly what you are missing in terms of understanding but at it's most basic it is just an optimised way of doing L3/L2 lookups on the device and then forwarding the packet. I'm assuming you have seen this doc but just in case -


Switching paths


2) MSFC - simply card on a supervisor that is responsible for the control plane or in less fancy terms building and mainting the routung table. Note it is not responsible for forwarding the actual data because on L3 switches that is done in hardware


3) TCAM - simply a fancy name for a specialised bit of memory that is optimised for table lookup eg. on the 6500 TCAM is used to store patterns for lookups -


6500 tcam


4) VRF - a virtual router. So you can have one physical device that has multiple vrfs. Each vrf has dedicated interfaces/subinterfaces on the physical router and each VRF has it's own routing and forwarding table. So a packet arriving on VRF1 interface will be routed based on VRF1s routing table. There may be other VRFs and a global routing table on the physical router but, unless specifically configured, VRF1 has no access to these.


Obviously the primary use of VRFs is with MPLS VPN technology.


If this has not answered your question perhaps you could be a bit more specific.


Jon

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