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MLS and CEF

Answered Question
Apr 28th, 2003
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I understand how MLS and its caching works on the switch. I also understand how CEF caching works as well.


However, what happens with the packets and frames when MLS is turned on on the switch, and the CEF turned on on the MSFC2 cards (routing module)?


Which one is faster? Which is more efficient? What problems could arise with having both of them turned on?


Thank you.


Correct Answer by t.baranski about 14 years 3 months ago

Interesting -- I've never used a 6509 with a SUP2 so I can't say for sure.


But per the CatOS documentation ( http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a008016399a.html ), it appears that there's a relationship between CEF and MLS on the Sup2, at least from a command standpoint. Apparently, what you're looking at with those "show mls" commands is actually CEF stats.

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t.baranski Mon, 04/28/2003 - 18:27
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This confused me as well a while back and it was explained to me as follows: in the case of a 6500 switch with a SUP1A, CEF only applies to packets that are sent to the MSFC for processing. When MLS is enabled, this is the first packet of each flow. So, the first packet is sent to the MSFC and CEF-switched while the remaining packets in that slow are switched by the PFC via MLS.


When a SUP2 or SUP3, however, the PFC itself can use CEF, so there's no concept of MLS -- all IP packets (with the usual list of exceptions -- see the documentation) are CEF-switched by the PFC.

spalislam Mon, 04/28/2003 - 21:14
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thanks for the info. However, let me ask you this?


My sup is sup II, and yes, I do have CEF enabled. However, show mls shows that mls is enabled as well, and I can see packet/frame manipulation and caching at MLS statistics.


I have heard form someone what you said before, that in SUP 2 and SUP 3 CEF is default and MLS is turned off, but why is then my mls turned on by default???

Correct Answer
t.baranski Tue, 04/29/2003 - 18:50
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Interesting -- I've never used a 6509 with a SUP2 so I can't say for sure.


But per the CatOS documentation ( http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a008016399a.html ), it appears that there's a relationship between CEF and MLS on the Sup2, at least from a command standpoint. Apparently, what you're looking at with those "show mls" commands is actually CEF stats.

spalislam Tue, 04/29/2003 - 20:54
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Actually, you are correct. I apologize. I was looking at the wrong place.


MLS stays on on the switch. However, on the MSFC card MLS is turned off in favor of CEF. And thanks for the link. When I get a chance I'll read the doc to see exactly what is happening.


Thanks again.

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