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Cut-through vs Store and Forward in ACI / NX-OS N9K

Hi there,

 

I am coming from an N5K - N7K setup, where I know that N5k regularly do cut through switching and N7K always store and forward.

That is relevant to know in regards to how broken frames (CRC errors) are discarded or spread through the fabric.

Now I know if I do VXLAN/EVPN on the N5K they become store and forward as well.

 

How is this with N9K in NX-OS and ACI-Mode? Is there a difference between spine and leaf? If they are cut-through, would we see broken frames to spread through the L2-domain as well?

Is there a differene on N9K NX-OS, if we do simple switching vs VXLAN/EVPN?

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VIP Engager

Hi @pille1234 

ACI (and I suppose the same applies in nxos mode while running VXLAN) is configured to work in store-and-forward, thus any CRC received will be dropped locally on the leaf. However, starting with 4.2.something (i am running 4.2.5k) you can actually see if the received corrupted frame is link local generated, or if they are stomped by other cut-through switch in the network. 

aci_crc_and_crc_stomped.png

This is the description of these counters from the help window:

 

CRC Errors (FCS Errors)
The number of valid size frames with Frame Check Sequence (FCS) errors but no framing errors.

FCS error count is the number of frames that were received with a bad checksum (CRC value) in the Ethernet frame. These frames are dropped and not propagated onto other ports.

Stomped CRC Errors (packets)
The number of corrupted frames that were cut-through switched.

 

Regarding Neuxs 9000 running NXOS, by default they run in cut-through. If you enable VXLAN they change the switching mode into store-and-forward.

 

Stay safe,

Sergiu

View solution in original post

1 REPLY 1
Highlighted
VIP Engager

Hi @pille1234 

ACI (and I suppose the same applies in nxos mode while running VXLAN) is configured to work in store-and-forward, thus any CRC received will be dropped locally on the leaf. However, starting with 4.2.something (i am running 4.2.5k) you can actually see if the received corrupted frame is link local generated, or if they are stomped by other cut-through switch in the network. 

aci_crc_and_crc_stomped.png

This is the description of these counters from the help window:

 

CRC Errors (FCS Errors)
The number of valid size frames with Frame Check Sequence (FCS) errors but no framing errors.

FCS error count is the number of frames that were received with a bad checksum (CRC value) in the Ethernet frame. These frames are dropped and not propagated onto other ports.

Stomped CRC Errors (packets)
The number of corrupted frames that were cut-through switched.

 

Regarding Neuxs 9000 running NXOS, by default they run in cut-through. If you enable VXLAN they change the switching mode into store-and-forward.

 

Stay safe,

Sergiu

View solution in original post

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