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New Member

C6K Frame Flow

Dear Network Professional Connection,

I would like to understand how a C6K (fabric-only) line card with two fabric connections and no classic bus interface (to my knowledge) communicates with a classic line card which comes without fabric access like a Flex-Wan card. I do know how fabric-enabled card work which offer both connection types.

Thanks in advance

  • Other Network Infrastructure Subjects

Re: C6K Frame Flow

The SFM which has a connections to the XBAR and the the 32 Gig BUS, all of these cards also connect to the DBus and RBus.

So if if a packet comes into a DFC enabled card and no local switching decision is made the packet will traverse to SFM and up the BUS to SUP module by doing this all ports in the switch copy the packet.

The results of the lookup will be sent down the RBus which instructs what ports should flush the packet.

This is all accomplished because of the modes you configure, you will notice that if you have a mix of classic and SFM cards then the mode drops to flow-through or truncated hence the forwarding engine (Sup2/PFC2) only sees 64 byte packet headers (truncated-headers) Without the SFM in the system, the 32 Gbps bus and forwarding engine will receive the entire packet from fabric-enabled linecards - this utilizes more of the bus capacity.


The key difference in the DFC-enabled system is that a switching decision is made on the input module instead of centrally at the Supervisor Engine.


New Member

Re: C6K Frame Flow

Thanks for the fast reply!

The only question/issue I would like to resolve is the point how the SFM connects to the chassis and indirectly to the other blades. From my knowledge the SFM does not connect to the legacy bus except to get power. Could you direct me to any pice of information about the D/R/C Bus connection of an SFM?

Thanks & have a great day

Re: C6K Frame Flow


Hope this helps, i got this info on a gold partnet training.

Also bear in mind the fabiric only cards punt traffic towards the SFM which in turn punts it onto system (normal) BUS via the Medusa.

A critical component of the local-switch implementation is the connection point between the local system and the SFM. In the Catalyst 6500, this function is handled by an ASIC called Medusa. This ASIC is the interface between the local bus and the crossbar.