Cat6k CEF256/dCEF256 modules

[Alexey Stytsenko]

Good day.

In Cisco Catalyst 6500 Architecture White Paper that when modules archtecture considered CEF256 and dCEF256 have more differencies that siply absence and presence of DFC module. See pics bellow:

                                                  CEF256

                                                  dCEF256

Besides it stated there in CEF256 module description that

By using this bus, a locally switched packet  (using a Distributed Forwarding Card (DFC) to determine the forwarding  destination) can avoid being transmitted over the chassis shared bus or  the crossbar switch fabric. 

But i thought that if we add DFC to CEF256 it will become dCEF256, but taking in consideration images adduced i am now totally confused.

Can smb clarify?

Tnx in advance!

[Jon Marshall · Accepted solution]

All of the below assumes a sup720 ie. a supervisor that supports connections to the crossbar switch fabric for transmission of packets.

A CEF256 module without an optional DFC card would need to use the shared 32Gbps bus to send the header of the packet to the supervisor for a forwarding decision. It is acting here as a CFC  (Centralised Forwarding Card). Once the result of that decision is sent back the whole packet can then be forwarded over the switch fabric connection.

If the CEF256 module had an optional DFC then it would not need to send the header to the supervisor it could make the decicion locally and then forward the packet.

Key thing to note though is that the CEF256 module has a connection to the 32Gbps shared bus so it can operate with or without a DFC ie. it is optional.

A dCEF256 module does not have a connection to the 32Gbps shared bus so it cannot use the supervisor to make forwarding decisions. So it has to make all the forwarding decisions locally on the card and this is why a DFC is mandatory ie. it comes integrated into the module.

In terms of operation a CEF256 module with DFC and a dCEF256 module act in the same way with regard to forwarding packets although you'll notice that in the diagrams above the dCEF module has 2 x 16 Gbps local bus. Both modules will do a lookup locally using the DFC and then forward the packet either -

1) to another port on the same module using the local bus

or

2) onto the switch fabric if the destination is on another module

This is the only way the dCEF module can operate whereas the CEF256 module without a DFC could use it's 32Gbps shared bus connection.

Hope that clarifies.

Jon

[Alexey Stytsenko]

So addition of DFC to module:

1) Will not make CEF256 module dCEF256 module

2) Will make CEF720 module dCEF720 module

Is it right?

[Joseph W. Doherty · Accepted solution]

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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.

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Posting

So addition of DFC to module:
1) Will not make CEF256 module dCEF256 module
2) Will make CEF720 module dCEF720 module
Is it right?

No, a DFC will convert a CEF256 to dCEF256, however these cards have a classic bus and single fabric connection while a "original/initial/only" dCEF256 card has no classic bus and dual fabric connections.

Yes, a DFC will convert a CEF720 to a dCEF720.  None of the CEF720 cards have a classic bus connection, but some only have one fabric connection while others have dual fabric connections.  Also, some are "originally/initially" have a a DFC, so they are "originally/initially" dCEF720 cards.

The "d" prefix, whether for CEF256 or CEF720 stands for "distributed" CEF forwarding.

[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 whatsoever (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

What's a bit confusing, there's CEF256 cards that have a classic bus and one (8 Gbps) fabric connections, and there's (one only? - i.e. the 6816-GBIC) dCEF256 with no classic bus connection but with dual (8 Gbps) fabric connections.  The former's architecture is shown at the top of your original posting, the later's at the bottom of your original posting.

The "dCEF256" 6816-GBIC card comes standard with a DFC.  The CEF256 cards, as also noted in the top of your original posting, may be provided with an optional DFC.

If a CEF256 card doesn't have a DFC, all forwarding decisions are made by the main supervisor.  As these cards have both classic and a fabric connections, either might be used for data transfer.  Which connection is used depends on the kind of other cards in the chassis and how chassis is configured.

When an optional DFC is added to a CEF256 card, it enables local forwarding, effectively converting the card from CEF256 to dCEF256.  However, the upgraded cards still have a choice of using either the classic bus or single fabric connector.  The "always" dCEF256 card, again, have the dual fabric connections; i.e. so the two card types (CEF256 => dCEF256 vs. dCEF256) still differ architecturally even though both will perform local forwarding.