distributed forwarding on catalyst 6500 with DFC

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Feb 6th, 2008
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Can you clarify for me the following: on cat6500 terms "local switching", "distributed forwarding" for fabric enabeled modules with DFC daughtercards refer to the local lookups, local header/ACL processing of the packets on the line modules itself to offload PFC and MSFC on a supervisor. Nevertheless, even the ingress and egress ports are on the same line card the packet still needs to go through the crossbar switching fabric. Or an opposite, the packet is buffered and switched from ingress to egress ports locally on the line card completely, thus not requiring access to the crossbar switching fabric at all.


regards

alex


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Amit Singh Wed, 02/06/2008 - 03:34
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Hi Alex,


With local switching the packet is locally processed on the lince card, it doesnot go to cross-bar fabric for processing. A packet is placed on the cross-bar fabric if a port on a line-card need to talk to the port on another lince card or sup. As far as the packet is on the same line card it doesnot need to go to the fabric. Please see the link below for more info.


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


HTH, Please rate if it does.

-amit singh



Joseph W. Doherty Wed, 02/06/2008 - 05:16
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Amit's reference has most of the details, but if this is all new to you, perhaps a simple understanding might be, 6500 chassis has, depending on line cards, two ways for data to flow between cards. Either via the classic bus, much like a hub, or via fabric, much like a switch. (Both might be utilized, again depends on which cards are installed.)


The 6500 also always has one central "brain" (the supervisor), but some line cards support a local "brain" (the DFC). The DFCs, when possible, off-load the supervisor from making all processing decisions.


You can, within limits, mix and match bus/fabric and usage of DFCs. However, DFCs are usually only utilized when there's an available fabric, since the fabric often provides more bandwidth and the DFCs more aggregate "CPU".

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