Software CEF vs Hardware CEf

[reetesh]

I'm really confused about software and hardware cef. Here is an extract from the study material from cisco "A router with Cisco Express Forwarding enabled uses information from tables that are built by the CPU, such as the routing table and the ARP table, to build hardware-based tables that are known as the Forwarding Information Base
(FIB) and adjacency tables. These tables are then used to make hardware-based forwarding decisions for
all frames in a data flow, even the first frame"!!! 

Here is an extract from the official cisco ccnp study guide "The general-purpose CPUs on software-based and hardware-based routers are similar and perform all the same functions; the difference is that on software-based routers, the general purpose CPU is in charge of all operations, including CEF switching (software CEF), and the hardware-based routers do CEF switching using forwarding engines that are implemented in specialized ASICs, ternary content addressable memory (TCAM), and NPUs (hardware CEF)"

It's really confusing. From the first extract the FIB and Adjacency are hardware tables which are built by the CPU.

The Extract from the official study guide says software CEF consists of the FIB and the adjacency tables.

Can someone clarity this concept please. Thank you in advance.

[MHM Cisco World]

friend dont confuse, the first think the router do is build table FIB and adjacency in CPU,
if the router is software-based then all forwarding done in CPU 
if the router is hardware-based then the CPU will copy tables (FIB and Adj.) to all ASIC card, and from there ASIC now do forwarding. 

that the different. 

[reetesh]

Thank you for your reply.
Still when we read the extract from cisco, in CEF, hardware forwarding
tables are ceated by the cpu regardless of traffic flows and all packets
switched using hardware.

What i can grasped is that the FIB & Adjacency tables reside in the data
plane and hence switched directly through hardware.

[MHM Cisco World]

you are right reside in the data plane BUT data plane which is run in hardware ACIS not run in software which is CPU.

I will ask you something here, which one is faster router or switch (L3SW) ??

[reetesh]

Thank you very much.

[MHM Cisco World]

You are so so welcome friend 

[reetesh]

Sorry i missed out your question.
Layer 3 switch is faster as switching is done in ASICS. (Hardware)

[MHM Cisco World · Accepted solution]

I search for ciscolive explain the SW CEF and HW CEF with more detail, check link below 
https://www.alcatron.net/Cisco%20Live%202015%20Melbourne/Cisco%20Live%20Content/Enterprise%20Networks/BRKARC-2350%20IOS%20Routing%20Internals.pdf

[cosmingrosu]

The official ENCOR Cert Guide is really confusing when explaining CEF:
For instance:
1. under Key topic: Cisco Express Forwarding, it says ...the difference is that on software-based routers, the general-
purpose CPU is in charge of all operations, including CEF switching (software CEF)...

but a bit later, under Key Topic: Software CEF it says: Upon receipt of an IP packet, the FIB is checked for a valid entry. If an entry is missing, it is a “glean” adjacency in CEF, which means the packet should go to the CPU because CEF is unable to handle it.

??? What does it mean the packet should go to the CPU? It said earlier, that the entire software CEF function is performed by the CPU!

2. under Hardware CEF, it keeps talking about the advantages of distributed forwarding architectures:

Why?? Am I to understand that Hardware CEF is found on distributed forwarding architecture devices only?

In the end I had to figure out all these concepts searching the web, so it's not okay at all

[Tyler McClure]

Currently reading the OCG and am having the same exact questions! Don’t forget to print out the 33 page errata

[Ramblin Tech]

What questions do you have regarding switching/forwarding?