CEF modes: central cef mode,dcef

[sarahr202]

Hi everybody

I am little confused about the location of FIB table and adjacency table in both cisco 6500 series and fixed ports switches .

In case of 6500 series switches , this is what my book says:

"Central CEF mode:The CEF FIB and adjacency tables reside on the route processor,

and the route processor performs the express forwarding. Use this CEF mode when

line cards are not available for CEF switching, or when features are not compatible

with distributed CEF."

That is what I understood from the above:

If line cards do no support CEF, then FIB and adjacency tables are built and are located on route processor( control plane ). Data plane operations are implemented in software and route processor performs those operations.

"

Some Cisco switches actually use different hardware to control the different planes. For

example, the Cisco Catalyst 6500 is a modular switch that uses the Multilayer Switch

Feature Card (MSFC) for control-plane operations, and the supervisor Policy Feature

Card (PFC) for the data-plane operations.

This is what I understood:

Supervisor module has  has MSFC and PFC. MSFC implements control plane operation where as PFC implements data plane operation.

But the first paragraph says in Central cef mode, both data plane and control plane operations are implemented by route processor.

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Fixed port catalyst switches: for e.g cat 3750 switch

I assume dcef is not possible because 3750 is fixed port switches. So only option is CEF in central mode.

Do these switches implement the Central mode cef in same way as mentioned above. i.e

"Central CEF mode:The CEF FIB and adjacency tables reside on the route processor and the route processor performs the express forwarding. Use this CEF mode whenline cards are not available for CEF switching, or when features are not compatible

with distributed CEF."

Or  fixed ports switches such as 3750 implement data plane operation in hardware where FIB tabe and adjacency table are maintained on Data plane.

thanks and have a great weekend.

[kamran_Roostaee]

When talk about Data plane means hardware components such as ASICs used by the switch for hardware switching but control plane is Layer 3 engine or route processor that is responsible for building the FIB table and adjacency tables and downloading this information to the data plane.

The Catalyst 3750 and other familes of MLS switch such as 3550, 3560 and 4500 do not have distinguishable control plane and data plane modules or line cards but in catalyst 6500 family of switches, the control plane hardware is easily distinguishable from the data plane hardware

In fixed port switches, switching, routing, ACL, QoS, and forwarding decisions are made on specialized ASIC that is

central to all interfaces and in some modular chassis, switching, routing, ACL, QoS, and forwarding decisions are made on the Supervisor Engine, we call it central CEF

With distributed switching, a centralized switching engine synchronizes Layer 3 forwarding, routing, and rewrite tables to local tables on distributed line modules that handles forwarding decisions independently, so individual line cards or ports make forwarding decisions without the aid of the centralized switching engine; In other words, switches using distributed switching place additional copies of the CEF FIB and adjacency table on line modules or interfaces for routing and switching of frames, so The main route processor of the switch is responsible for generating a central master FIB and adjacency table and distributing these tables out to each dCEF line card and Each line card that supports dCEF has its own dedicated hardware-based L3 routing engine and CEF route cache, allowing for multiple L3 data plane operations to be performed simultaneously within a single chassis-based system

[sarahr202]

Hi Kamran.

Let focus on cef in central mode.

CEF-based Catalyst switches support one of two methods of hardware switching at Layer 3:

• Centralized switching—Centralized switching carries out forwarding decisions on a specialized ASIC that is central to all interfaces of a Layer 3 switch. With centralized switching, routing, ACL, QoS, and forwarding decisions are made on the Supervisor Engine in a modular chassis or by Layer 3 engines in fixed port density Layer 3 switches. As a result, all frames to be routed or switched must pass through the centralized engine via a fabric or bus. Furthermore, with centralized switching, the hardware-switching performance of the Catalyst switch is based on the central forwarding engine and the fabric or bus architecture. Figure 9-3 illustrates centralized switching, logically. Note in Figure 9-3 how frames must pass through the centralized switching engine.

Fixed port switches:

Is following statement correct?

Routing,ACL QOS and forwarding decisions are made by route processor( layer 3 engine) . FIb and adjacency tables are implemented using TCAM.

Modular switches:

These switches are the ones that can use centralized switching.

Catalyst 4500 family of switches and the Catalyst 6500 family of switches without the use of Distributed Forwarding Cards (DFC)

Is following statement correct?

Routing ,acl,qos and forwarding decisions are made by supervisor engine.  FIb tables and  adjacency tables are implemented using tcam.

NOTE:  The confusion arises because I have seen another book which shows FIB  and adjancency tables implemented in software in the context of CEF in central mode. 

thanks and have a great weekend

[sarahr202]

I found some answers

Fixed port layer 3 switches:

Such switches use cef in central mode. The key word central signifies the layer 3 switching engine and fib and adjacency tables are central to all ports of the switch.

Router processor ( control plane) generates FIB and adjacency tables and download them onto TCAM on data plane.

Cisco Catalyst switches such as 4500 ,6500 can also use the Cef in central mode  which is contigent  upon the line cards, supervisor engine and whether switching fabric module are used or not.

thanks and have a great weekend.