L2 Store and Forward & Cut-through

[oborrero29]

Greetings, I've been getting ready for the CCENT exam and I just came across the content for the different types of switching. Is there a way to configure one or the other in terms of cut through vs store and forward on a 2960? Or is this something that's hard coded into the switch itself?

[Arumugam Muthaiah]

Hi,

The switch 2960 always uses the store-and-forward method: complete packets are stored and checked for errors before transmission.

There is no need to configure the switching method. Most of the access switches will support "store-and-forward"

Example:

Store-and-forward

• Catalyst 2960 and 2960-S
• Catalyst 3750-X and 3560-X

Cut-through

• The Cisco Nexus 5000 Series access-layer switch is an example of a low-latency cut-through single-stage fabric implementation

Regards,

Aru

*** Please rate if the post is useful ***

[Arumugam Muthaiah]

I would like to add below as well,

Starting in 5.1(3)n1(1), we have the ability to change the default behavior to store and forward with the following global command: hardware ethernet store-and-fwd-switching

Refer:

http://www.cisco.com/c/en/us/products/collateral/switches/nexus-5548p-switch/qa_c67-618605.html

Regards,

Aru

[oborrero29]

Thank you so much Amuthaia! And what about fragment free, where would you see that?

[Arumugam Muthaiah]

Hi,

• The first switches (made by Kalpana, who was later bought by Cisco). Kalpana was the first, and they did it with custom proprietary hardware.

• The store-and-forward was much higher latency through the switch. Then Cisco came out with "Fragment Free" switching - meaning that the switch would accept the first 64 bytes, evaluate it, then forward it.

• By getting at least 64 bytes, the switch could guarantee the the frame was not a runt (runt is a frame < 64bytes, the minimum permissable Ethernet frame size), and could make sure that the addresses were not corrupt.

• I remember that the catalyst 1924 is set for fragment-free switching which was very old switch.

• But presently the switches are operating with Store-and-forward and cut-through switching

Refer

https://supportforums.cisco.com/discussion/9796971/fragment-free-switch 

Regards,

Aru

*** Please rate if the post is useful ***