CEF and MLS

Harold Arley Morales has asked an interesting question:

What's the difference between Cisco Express Forwarding and Cisco MLS? Is Cisco's implementation of MLS standardized?

CEF is a routing table lookup mechanism. Instead of doing a lookup in the main IP routing table (displayed with the show ip route), the router does a lookup in a fully computed non-recursive version of the IP routing table (Forwarding Information Base - FIB) with layer-2 next-hop information attached to it (adjacency table).

MLS is a caching mechanism (similar to Netflow) that offloads layer-3 processing from the routing component into layer-2 ASICs that cannot perform full-blown layer-3 switching. When the layer-2 engine detects a single IP packet traversing multiple VLANs, the MLS populates the cache with the flow details and the subsequent packets belonging to the same flow (same source/destination IP addresses and port numbers ...) are switched without going through all the layer-3 mechanisms (for example, access lists). The Multilayer Switching Overview document gives you additional details.

The MLS uses a proprietary protocol (MLSP) through which the layer-2 switches identify routers.

This article is part of You've asked for it series.

Update 2008-12-08: Ofer Granit sent me the following information: according to Troubleshooting IP Multilayer Switching document, Supervisor Engine 2 and Supervisor Engine 720 no longer use MLS but rely exclusively on CEF to perform layer-3 forwarding.

7 comments:

  1. Harold Arley Morales06 June, 2008 18:15

    Thank you very much Ivan...

    ReplyDelete
  2. From the Multilayer Switching Overview:
    "Enabling IP accounting on an MLS-enabled interface disables the IP accounting functions on that interface."
    My english is not enough to understand this. :D

    ReplyDelete
  3. Hi Ivan,

    Thank you for the post. Is MLS still used today. I was under the impression that newer CEF demand based mechanism replaced MLS specifically on platforms like the 6500 and the 7600. Thanks

    ReplyDelete
  4. I would need to check with our switching gurus, but you're most probably correct.

    ReplyDelete
  5. mls is diabled by default and cef is enabled by default in the newer cat6k switches

    ReplyDelete
  6. I learnt from your nice document that SUP2 doesn't support MLS and uses another form of protocl "IP Unicast Layer 3 Switching ". But what about SUP 2T. Does SuP-2T also not support MLS ?

    ReplyDelete
  7. Hi,


    I found that Sup2 is now end of sale and also going to be end of support in feb, 2012. It was replaces by SUP 720. But according to cisco some version of sup 720 are also being replaced by sup720 xl and also by supervisor engine sup 2T.


    http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/prod_end-of-life_notice0900aecd80423d31.html



    http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/eol_c51-697975.html



    Regards,

    Uma

    ReplyDelete

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Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.