What is the difference between DFC and CFC?

[Soon Kwan Kwon]

I know the DFC and CFC's roles, separately.

But, after reading the datasheet or related forum, I wondered why CFC was existed fundamentally.

CFC is a centralized forwarding card.

So, I guess CFC's role is for Supervisor!

==================================

Anyway, back to the point, the thing that I want to know is the difference between DFC and CFC!

Regards,

[leshkamal.kumar1 · Accepted solution]

https://supportforums.cisco.com/document/85621/understanding-msfc-pfc-and-dfc-roles-catalyst-6500-series-switch#Multilayer_Switch_Feature_Card_MSFC

Distributed Forwarding Card (DFC)

The Catalyst 6500 architecture supports the use of Distributed Forwarding Cards (DFC). Distributed Forwarding Card is a combo daughter card comprising a MSFC and PFC used by a fabric enabled Cat6500 linecard to perform distributed switching. DFCs are located in linecards, not in Supervisors.

A DFC is used to hold a local copy of the forwarding tables (constructed by the MSFC) along with Security and QoS policies to facilitate local switching on the linecard. The DFC3A is available as an option on CEF256 and CEF720 based linecards. The DFC3B and DFC3BXL were introduced for linecards to operate with the Supervisor 720 equipped with PFC3B and PFC3BXL. The last generation of DFC, the DFC3C, is available as an option on the CEF720 based linecards but are integrated on the latest generation linecards, the WS-X6708 and WS-X6716.

It is important to note that there are some operational considerations that can impact the ability of the Catalyst 6500 system to provide specific QoS features. This can happen when you mix different generations of PFC's and DFC's together. The rule is that the system will operate at the lowest common feature denominator

Table 3.  DFC/PFC Operation 

	PFC3A	PFC3B	PFC3BXL	PFC3C	PFC3CXL
DFC3A	Normal Operation	PFC3B operates as a PFC3A	PFC3BXL operates as a PFC3A	PFC3C operates as a PFC3A	PFC3CXL operates as a PFC3A
DFC3B	DFC3B operates as a DFC3A	Normal Operation	PFC3BXL operates as a PFC3B	PFC3C operates as a PFC3A	PFC3CXL operates as a PFC3B
DFC3BXL	DFC3BXL operates as a DFC3A	DFC3BXL operates as a DFC3B	Normal Operation	PFC3C operates as a PFC3BXL	PFC3CXL operates as a PFC3BXL
DFC3C	DFC3C operates as a DFC3A	DFC3C operates as a DFC3B	DFC3C operates as a DFC3B and PFC3BXL operates as a PFC3B	Normal Operation	PFC3CXL operates as a PFC3C
DFC3CXL	DFC3CXL operates as a DFC3A	DFC3CXL operates as a DFC3B	DFC3CXL operates as a DFC3BXL	DFC3CXL operates as a DFC3C	Normal Operation

The primary MSFC3 will calculate, then push down a FIB table (Forwarding Information Base) giving the DFC3x its layer 3 forwarding tables. The MSFC3 will also push down a copy of the QoS policies so that they are also local to the line card. Subsequent to this, local switching decisions can reference the local copy of any QoS policies providing hardware QoS processing speeds and yielding higher levels of performance though distributed switching.

Benefits of DFC

Performance is the biggest and most obvious reason to implement DFCs. You move from a 30 Mpps centralized forwarding system anywhere up to a 400 Mpps distributed forwarding system. This forwarding performance is for all L2 bridging, L3 routing, ACLs, QoS, and Netflow features, i.e., not just L3.

The performance benefit of a DFC is most applicable when you use the 67xx series modules. This is because these modules have enough ports and bandwidth to generate much more than the 30Mpps centralized forwarding engine has available. A 67xx-series module without a DFC is subject to the same centralized performance characteristics of all other centralized forwarding modules.

DFC also minimize the impact that a classic module has in a system. Classic modules do affect the centralized forwarding performance of a system, limiting the maximum centralized forwarding rate to 15Mpps. Modules enabled with DFCs have their own forwarding engine and are not subject to this performance degradation. If a classic module used, the inclusion of a DFC mitigates any performance issues/concerns. Any non-DFC modules are still subject to the available 15 Mpps of forwarding available when a classic-module is present.

Packet Forwarding

Packet Forwarding is done on the ingress forwarding engine. Therefore, packets coming into the ports on the Sup720-3B will have forwarding done on the PFC3B of the Supervisor. Packets coming into ports of line cards with DFC3s will have the forwarding done on the DFC3. Packets coming into ports of line cards with CFCs will have the forwarding done on the PFC3B of the Supervisor. The MSFC3 only does forwarding in the cases where the PFC3 or DFC3 cannot make the forwarding decision. Some of these cases include when traffic has IP Options set, when ACLs are applied to an interface but the ACL is not programmed into the ACL TCAM for some reason, when packets have TTL expiration, when packets hit an ACE with the "log" keyword, and others.

Centralized Forwarding Card (CFC)

CFC is a centralized forwarding card for the switching modules which makes IPv4 Routing over the PFC. CFC does not do local forwarding, the forwarding is done by the PFC in the Supervisor. As the forwarding is centralized, the PFC performance, FIB entries, ACL lables are shared among the line cards that uses the Supervisor PFC for forwrding. WS-F6700-CFC is the CFC card used on WS-X67xx Ethernet Modules. This daughter card is supported only by the Supervisor Engine 720.

Note: CFC or the Centralized Forwarding Card was introduced along with the CEF720 modules. It provides centralized connectivity to the supervisor for look-ups and results. Though the switch fabric is used for the data, but the CFC is responsible to send a look-up request from the Supervisor and then get those results back.

[Soon Kwan Kwon]

Thank you,

But, I already read it.

<DFC>

A DFC is used to hold a local copy of the forwarding tables (constructed by the MSFC) along with Security and QoS policies to facilitate local switching on the linecard.

<CFC>

CFC provides centralized connectivity to the supervisor for look-ups and results.

If I try to understand fundamental reason between DFC and CFC, it will be difficult one.

,, actually I don't understand what is the difference between them even though I read a article like you mentioned.

Please contact me.

Regards,

[Joseph W. Doherty · Accepted solution]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

If I try to understand fundamental reason between DFC and CFC, it will be difficult one.

,, actually I don't understand what is the difference between them even though I read a article like you mentioned.

The fundamental difference is, the line card is "stupid" with a CFC, all forwarding logic is provided by the supervisor, while the line card is "smart" with a DFC, forwarding logic is conducted on the line card, the supervisor is bypassed.

The big advantage using DFCs is it greatly extends the forwarding performance of the chassis.  For example, a sup720 offers 15 or 30 Mpps, but a DFC3 adds 48 Mpps (per line card).

[Soon Kwan Kwon]

Thank you expert!

Then, I have a question.

For example,

If I use SUP-720-3B and WS-X6724-SFP(CFC), 6724 can help Sup-720 by giving centralized data?

According to leshkamal.kumar1's URL, there is a sentence below.

Note: CFC or the Centralized Forwarding Card was introduced along with the CEF720 modules. It provides centralized connectivity to the supervisor for look-ups and results. Though the switch fabric is used for the data, but the CFC is responsible to send a look-up request from the Supervisor and then get those results back.

Among the sentences, I'm wondering "CFC provides to the supervisor for look-ups and results."

I can understand it -> It means that CFC helps sup to find out data which is requested by Sup-720.

Can you explain it more easily.. ?

Regards,

[Masoud Pourshabanian · Accepted solution]

Hello,

It is just a simple explanation.

SUP720 has 2 internal components. MSFC and PFC.

MSFC is kind of memory based CPU and PFC is hardware base( ASIC-based forwarding engine).

Some processes can be handled by hardware such as routing or QOS or ACL so they are processed by PFC

MSFC handes layer 2 and routing protocols.( receives routing updates and puts them in RIB table then makes FIB)

MSFC builds FIB table and downloads it to PFC if you active CEP by IP CEF command. Then PFC can forward packets much faster since its hardware based.

****************

There are two different line cards inserted into 6500 or 7600.  DFC, CFC.

CFC cards does not process any packets or frames(dumb) . CFC cards send the packets to PFC to process. They use central processor

DFC cards have ASIC hardware and able to get a copy of PFC so they can process locally.

Hope it helps,

Masoud

[Soon Kwan Kwon]

Excellent explanation !!

[Masoud Pourshabanian]

I am glad it was helpful.

Masoud

[Soon Kwan Kwon]

Can I ask you last one question?

Thanks to you guys' excellent comments, I can understand more easily than past.

However, I think CFC seems to be not useful than DFC.

That is, CFC which is put in module such as 6748-SFP needs to be replaced to DFC!

Why 6748-SFP obstinately use CFC instead of DFC?

Last question!

Thank you^^

[Masoud Pourshabanian]

Hello,

I am not sure if I understood your question correctly.

CFC, DFC3 daughter boards can be installed on WS-X6748-SFP.

If you order WS-X6748-SFP, it comes with WS-F6700-CFC(installed by default). It gives you up to 30 Mpps per system because it needs to communicate with PFC to make decision.

You have an option to upgrade it to DFC3(by installation of DFC3 daughter board). It gives you up to 48 Mpps sustained per slot since it is able to make decision locally.

Let me know If I understood you wrong.

Masoud

[Soon Kwan Kwon]

Thank you Masoud!

I mentioned by my fault, I should have mentioned you 6724! not 6748.

So, mine(6724-SFP) has only CFC module!!.. (So I asked just before question)

                                          <WS-X6724-SFP>

Hmmm

I'm wondering what is "per system" meaning?

You mentioned that CFC gives up to 30 Mpps per system

So I've understood that "Ah~, without CFC, line cards can transmit the packet lower than 30Mpps!"  <-- Can I understand like this?

Regards,

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

"per system" means all the non-DFC line cards "share" the supervisor's forwarding capacity.  (Also, 30 Mpps is only for fabric cards, non-fabric cards share 15 Mpps.)

A DFC3's 48 Mpps is only available to traffic that enters a port on such an equipped line card.  I.e. A DFC's performance isn't shared with any other traffic on the system, but by not using the supervisor's capacity, it leaves more of the supervisor's capacity for non-DFC line cards.

Normally, 30 Mpps supports only about 20 Gbps of Ethernet bandwidth, for minimum size packets, but PPS requirements decrease as packet size increases and, for fabric line cards, the 30 Mpps is available for all packet sizes (many switch's PPS capacity decreases as packet size increases).  This means, a sup720's 30 Mpps can support much, much more than 20 Gbps, for "typical" packet sizes.

In "typical" Enterprise usage, user edge 6500s, and sometimes distribution 6500s, often work just fine with gig line cards even without any DFCs.

PS:

BTW, since you don't always "need" DFC performance, and they are an expensive component, often a CFC equipped line cards is the "right" choice.

PPS:

Also BTW, the later VS-S720 offers a higher Mpps rate, and if you have two installed in a chassis, the 2nd supervisor is also used.  So, buy replacing the chassis supervisor, the whole chassis non-DFC cards can take advantage of the additional forwarding capacity.

[Soon Kwan Kwon]

Merry Christmas !! 

Thank your for the great answer! ^^

[Masoud Pourshabanian]

There are several delay types involved in network throughput. Delay of using central PFC is not comparable with other delays so if you have low traffic in your network, this delay is negligible; however, if you have high traffic, you should be concern

It depends what kind of traffic and how much traffic you have in your network.

Which CFC line card and average packet size, 6500 with sup720 offers about 150 to 200G bandwidth shared among all cards. (only one card  can not have this much bandwidth due to switch fabric limitation,one card only 20G to 40G)

average packet size =750B

30Mpps* 750B*8=180 Gbps bandwidth

Voice packet size 64B

30Mpps*64*8=about 15Gbps

If you have more traffic in your network, you should be concern.

Masoud

[Joseph W. Doherty]

Just in case you noticed Masoud has 15 Gbps for 64 byte packets, and I noted 20 Gbps Ethernet bandwidth for minimum size packets, both for 30 Mpps, and if you wondered about the difference, Masoud's bandwidth is L3 only, mine includes Ethernet L2 overhead bandwidth (the bandwidth on a wire).