CEF and MLS on 6500

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Nov 5th, 2008
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Hello Everyone,


I have a 6509 switch on my network. I am a bit new to the L3 switching technology. But I was under the impression that MLS(route caching) has been replaced by CEF. I have a sup720 installed. I issued a show ip cef and got to look at the CEF table. But I was also able to look at show mls cef with some entries.

* So is my switch running CEF and MLS(route-cache)?


* why is there a Layer 2 forwarding engine , when we already have a L3 forwarding engine which does the ASIC based manipulation ( the o/p from sh mls stat is pasted below)

show mls statistics


Statistics for Earl in Module 5


L2 Forwarding Engine

Total packets Switched : 443298873549


L3 Forwarding Engine

Total packets L3 Switched : 443005602249 @ 56324 pps


After noticing this output I am inclined to think that the l2 forwarding engine is the dataplane, the L3 forwarding engine is the control plane and the Software L3 engine is the RP(route processor) Am I right?


* Is TCAM used to store the FIB and the ADj Table that CEF uses, so everytime we say a CEF lookup its actually a TCAM lookup ( FIB lookup at the IP-prefix region and then using the next hop IP another TCAM lookup at the adjacency region for the rewrite info)returning the mem pointers to the relevant match?



Please let me know your thoughts on this.


Thanks,

Deesh


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Amit Singh Wed, 11/05/2008 - 05:16
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Deesh,


* So is my switch running CEF and MLS(route-cache)?


Your switch is MLS using CEF for route-caching. I know its kindaa contradictory on the MLS and CEF terminology but its just the command " MLS " that you use to turn on CEF on Cat6500 platform.


2. The layer 2 forwarding engine is the one which has your all Adjacency tables stored in. L3 forwarding engine only uses the FIB.


3. Yes TCAM is used to store all the FIB and ADJ table which are a part of PFC itself which has all these hardware ASIC fixed on to it.


-amit singh

Jon Marshall Wed, 11/05/2008 - 05:24
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Amit


Sorry, this keeps happening today, ie. my post turns up about 5 mins after yours.


Jon

Jon Marshall Wed, 11/05/2008 - 05:23
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Deesh


MLS uses CEF to switch packets. Prior to Supervisor 2 MLS did not use CEF, so they are used in conjunction with each other.


The PFC card is the layer 2 and layer 3 forwarding engine ie. it is is responsible for the data plane on the 6500. The PFC is repsonsible for switching both L2 and L3 packets.


The MSFC is the control plane card. This cards primary responsibilty is to establish routing neighborships, exchange routing information and build an forwarding table that can be used by CEF.


The FIB s built by the MSFC and pushed to the PFC which stores this information in TCAM.


You may want to have a look at this whitepaper on 6500 architecture which goes into a lot more detail -


http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/prod_white_paper0900aecd80673385.html


Jon

Amit Singh Wed, 11/05/2008 - 05:28
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No problem Jon. Keep up the good work :-)



prakadeesh Wed, 11/05/2008 - 05:49
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Thank you Jon and Amit :). I will go thru the white paper. Thanks again for your time



-deesh


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