Catalyst 9k Switching Modes ( Cut-Through vs Store-and-Forward)

[chira.cipri@gmail.com]

Hello community,

What is the default switching mode on the Catalyst 9K series of switches ? Store-and-forward or Cut-through ? Can the switching mode be changed from the configuration ?

I have searched through Cisco docs and I can't find anything on the Cat 9Ks. I can only find information about the Nexus line of switches, and it seems a lot of them support cut-through switching, which is even the default switching mode at least for the Nexus 3Ks and Nexus 9Ks.

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus9000/sw/7-x/layer2/configuration/guide/b_Cisco_Nexus_9000_Series_NX-OS_Layer_2_Switching_Configuration_Guide_7x/Configuring_Switching_Modes.html

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus3000/sw/layer2/503_U3_1/b_Cisco_n3k_Layer_2_Switching_Config_503_U31/b_Cisco_n3k_Layer_2_Switching_Config_503_U31_chapter_01000.html

[ngkin2010]

The only document that I can find for Catelysi 9000 series is :

The device sends packets between any combination of ports, based on the destination address of the received packet. Using the MAC address table, the device forwards the packet only to the port associated with the destination address. If the destination address is on the port that sent the packet, the packet is filtered and not forwarded. The device always uses the store-and-forward method: complete packets are stored and checked for errors before transmission.

https://content.cisco.com/chapter.sjs?uri=%2Fsearchable%2Fchapter%2Fcontent%2Fen%2Fus%2Ftd%2Fdocs%2Fswitches%2Flan%2Fcatalyst9300%2Fsoftware%2Frelease%2F17-1%2Fconfiguration_guide%2Fsys_mgmt%2Fb_171_sys_mgmt_9300_cg%2Fadministering_the_device.html.xm...

[Martin L]

I read that Cisco dropped other methods some time ago and only supports the cut-through switching method for all switching platforms nowadays.  this is the default switching mode and cannot be change. 

you can change, however,  "the way" packets are forwarded thru Cisco device and between ends via changing  CEF switching /processing methods.(although some options are limited as well). I think you can still turn off CEF and go legacy fast-switching method.

Regards, ML
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[Scott Hodgdon · Accepted solution]

chira.cipri@gmail.com ,

Catalyst 9K platforms use only Store-and-Forward. You can see this in the packet walk slides of the Cat 9K architecture sessions on ciscolive.com. Go to the on-demand library and search for:

• Catalyst 9000 Series Access Switching Architecture - BRKARC-3863 (Barcelona 2020), Minhaj Uddin Presenter
• https://www.ciscolive.com/c/dam/r/ciscolive/emea/docs/2020/pdf/BRKARC-3863.pdf (if you can go directly there)

Slide 66 has the Unicast packet walk and shows the packet being buffered while the UADP ASIC operations take place. All Cat 9K have UADP ASICs, and so they all use this same mechanism.

Cheers,
Scott Hodgdon

Senior Technical Marketing Engineer

Enterprise Networking and Cloud Group