6500 Architecture query... do you need a DFC to use the switch fabric?

[twhittle1]

Hi All,

I am reading throught the Cat6500 architecture white paper on cisco.com:

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

It's a really interesting read.

However I've got a query which I'm hoping someone might be able to enlighten me on.

Different generations of linecards have different connections they can use to the switch; classis, CEF256, CEF720 etc. the CEF linecards have fabric connections. My question is this:Does the line card need to have a DFC, and use distributed forwarding, to utilise it's switch fabric connections?

I.E if I had several different types of line cards, can they all use different connections? Classis line cards would use the shared 32GB Bus, Cef256 cards would use the 8Gb fabric and CEF720 cards would use the 20GB Fabric? And do they require the DFC to do this?

Or does the DFC only affect whether the line card does a lookup with the supervisor and it can use the fabric regardless?

Looking forward to your replies!

Many Thanks,

Tom Whittle

[more_jazZz_2 · Accepted solution]

Hi Tom,

Your last statement is right "DFC only affect whether the line card does a lookup with the supervisor and it can use the fabric regardless", for instance if you use Sup720 you can use classic (32 Gb bus) card, an 8 Gb fabric (CEF256) card and 40 Gb fabric (dCEF720).

Prove answer you can find here http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/prod_qas09186a0080159963.html

Best regards,

Zak.

[twhittle1]

Brilliant many thanks Zak. Thanks for the quick response

Regards,

Tom

[Joseph W. Doherty]

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Posting

Just want to emphasis that although a DFC isn't needed to pass data across the fabric, a DFC might be necessary to fully exploit the bandwidth of the fabric.  Also, different non-fabric and fabric cards have different transfer "modes", also impacted by DFCs.

Cisco Catalyst 6500 Architecture: Bus Switching Modes

There  are three switching modes used by the BUS and fabric ASICs present on  CEF256 and CEF720 line cards. These modes determine the header format  that is used to transmit the data across the DBus and communicate with  other CEF256 and CEF720 line cards. These modes do not apply to line  cards that use a DFC. These modes are discussed in more detail below.

Flow-Through Mode

This  mode of operation is used by CEF256 modules when there is no crossbar  switch fabric present. It enables CEF256 modules to operate as if they  were classic line cards. In flow through mode, the entire packet (header  plus data) is forwarded by the line card to the supervisor for  processing. This mode does not apply to CEF720 modules. When flow  through mode is used, performance levels up to 15Mpps can be achieved.

Compact Mode

The  compact mode of operation requires a crossbar switch fabric to be  present in the system. This can be provided by a switch fabric module,  Switch Fabric Module 2 or a Supervisor Engine 720. All modules in the  chassis must be fabric enabled (i.e. CEF256, dCEF256, or CEF720) for the  switch to operate in Compact Mode. Classic line cards installed in the  chassis will negate the ability of the switch to run in Compact Mode. In  Compact Mode, only the header is passed over the DBus to the  supervisor. The Header is compressed prior to being placed on the DBus  which increases the bandwidth available for header transmission. The  data portion of the packet is transmitted over the crossbar switch  fabric channels. In this mode of operation, the switch can achieve  centralized performance of up to 30Mpps independent of packet size.

Truncated Mode

Truncated  Mode is used when CEF256 and/or CEF720 line cards are installed in a  chassis with a classic line card, but a crossbar switch fabric is  present. In this mode, classic line cards will transmit both the header  and the data portion of the packet over the DBus. CEF256 and CEF720 line  cards will only transmit headers over the DBus and transmit the data  portion of the packet over the crossbar switch fabric.

Truncated  mode results in centralized forwarding rates up to 15Mpps. In Truncated  Mode, since the CEF256 and CEF720 line cards use the crossbar switch  fabric to transmit data, overall aggregate bandwidth can actually be  higher than the 32-Gbps shared bus capacity.

Performance  of DFC enabled line cards are not affected by truncated mode and  performance remains the same regardless of the line-card mix in the  chassis.

Also see http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801c6652.shtml.