[c-nsp] do i *need* DFCs on the 6500?

[Phil Mayers]

Ben Steele wrote:
> Unless you are hitting a cam limit on any of your resources on your SUP(very
> possible if you are exporting netflow) OR you are congesting the crossbar
> fabric(sh fabric util) which is pretty unlikely when you are talking a 24G
> linecard on a 40G fabric connection then you probably won't see any
> difference putting a DFC on a 6724

That depends completely on what other cards are on the box, what their 
offered forwarding load is, and whether they have DFCs.

> 
> Remember these chassis are a hardware only based forwarding solution, so all
> your doing with a DFC is moving cam/asic resources off the sup, so in
> regards to your specific questions unless you have filled all your QoS
> queues on the sup you are going to see nothing more on the DFC, also the sup
> does (from memory) up to 100-200m pps in ipv6, I don't believe for a moment

No. The PFC3 does 30Mpps IPv4 (and 15Mpps IPv6 I think). A DFC3 does 
48Mpps IPv4 (and 24Mpps IPv6).

http://www.cisco.com/en/US/products/hw/switches/ps708/products_qanda_item09186a00809a7673.shtml

A fully-populated and fully-DFCed 6509 does 400Mpps IPv4 or 200Mpps IPv6 
(well, actually 192Mpps - 24x8 linecards). In this configuration, the 
PFC does very little.

It's worth noting that a 6724 doing 64-bytes packets on all ports offers 
~47Mpps forwarding load - well in excess of the PFC capacity. A chassis 
full of 6724s without DFCs at 10% load with 64-bytes packets also 
exceeds the PFC capacity.

Obviously these are worst-case numbers but illustrative of the problems 
you can get yourself into if you don't capacity plan well.

It's worth noting that some linecards have different (i.e. more 
flexible) rx & tx queueing methods with a DFC versus the CFC.

There's also the bus-stall issues, which go away (supposedly) with a DFC 
installed since they're not connected to the bus.

> you are even remotely close to this, and the global ipv6 routing table is no
> where near the cam limit for that either, by the way is your SUP an XL? does
> the DFC's on the 10G's match the sup or have they fallen back to the lowest
> common configuration?

I'm not sure why you mention CAM limits, but it's worth noting that DFCs 
do not help with FIB CAM at all, since they hold a copy of the PFC FIB.

Personally we get DFCs on everything since we're using plain -3B (or -3C 
not) rather than XL, and the cost of the DFC is a pretty minimal 
percentage of the linecard for the future-proofing.

We've also seen software bugs manifest on CFC cards in the past; this 
implies to me that Cisco "prefer" DFC chassis. Similarly some of the new 
linecards e.g. 6708/6716 are DFC-only. I suspect that will be the case 
going forward.