Recommendation using "Forward Error Correction" on Cat9600 & Nexus9K.

[mhiyoshi]

Dear all,

Currently I have been investigating FEC recommendation for Catalyst9600 and Nexus9000.
The following links are related URL.

[Understanding FEC and Its Implementation in Cisco Optics]
https://www.cisco.com/c/en/us/products/collateral/interfaces-modules/transceiver-modules/implementation-optics-wp.html

[Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide 8.x]
https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus7000/sw/interfaces/config/cisco_nexus7000_interfaces_config_guide_8x/config-fec-on-optic-modules.html

[25GE and 100GE – Enabling Higher Speeds in Enterprise with Investment Protection White Paper]
https://www.cisco.com/c/en/us/products/collateral/switches/catalyst-9000/nb-09-25ge-100ge-wp-cte-en.html

In my understanding firstly each QSFP support the FEC like this and if both end support FEC OFF(NO-FEC), link state is UP
however if both end support FEC-AUTO or Enable as CL91(RS-FEC), then the FEC is functioning with data packet delay wright?

QSFP-100G-SR4-S (70/150m) === RS-FEC (CL91) or NO-FEC
QSFP-100G-LR4-S (10km) === NO-FEC
QSFP-40/100-SRBD (70-150m)=== NO-FEC
QSFP-100G-AOC (1-30m) === RS-FEC (CL91) or NO-FEC
QSFP-100G-SM-SR (2km) === RS-FEC (CL91) or NO-FEC
QSFP-100G-CWDM4-S (2km) === RS-FEC (CL91) or NO-FEC
QSFP-100G-PSM4-S (500m) === RS-FEC (CL91) or NO-FEC
QSFP-100G-CU (1-5m) === RS-FEC (CL91) or NO-FEC
etc...

So actually are there any recommencation whether both end have to enable this FEC feature or just FEC-OFF.
For example...

If WAN link => FEC is recommended?
If Directly connected => FEC is not necessary to enable?

I appreciate if you can let me know any comment, thank you in advance.

Best Regards,

Masanobu Hiyoshi

[M02@rt37]

Hello @mhiyoshi 

In general, it is recommended to enable FEC on both ends of a link, especially for longer links or links with higher data rates, as this can help improve the reliability and performance of the link. However, the specific FEC mode and settings to use will depend on the specific equipment and link characteristics, as well as any applicable standards or regulations.

For example, for optical links using the 100GBASE-LR4 or 100GBASE-SR4 standards, the use of FEC is mandatory, and the FEC mode and settings are specified by the standard. On the other hand, for shorter copper links or for links using other standards, FEC may not be necessary or may be optional.

[Joseph W. Doherty]

"I appreciate if you can let me know any comment, thank you in advance."

This is a "dam if you do and dam if you don't" situation.

Basically, FEC includes additional information to allow you to recover from some transmission errors.

Which sound great, except it comes at the expense of using additional bandwidth.

So it boils down to, for each possible cases, which is better for you, i.e. consume additional link bandwidth to avoid dropping a packet sometimes, or use the least amount of bandwidth but will likely have a higher overall packet drop rate.

BTW, understand, assuming all your connections are to "spec", part of that spec is expected error rates, which are, generally, very, very small to begin with.  I.e. I would not advise using FEC to make up for below spec links (although it might help), but more for guaranteeing more consistent latency (a tad slower with FEC), as less packets would need to be retransmitted.

[mhiyoshi]

Hi M02@rt37 and @Joseph W. Doherty

Thank you for your precious comment! Basically FEC is mandatory however it is
especially for longer links with optical but for shorter copper links is optional right?

And please let me make sure of "100GBASE-LR4" mentioned before, I think according to the above link,
Cisco QSFP-100G-LR4-S is not support for FEC right?

Best Regards,

[Joseph W. Doherty]

"Basically FEC is mandatory however it is especially for longer links with optical but for shorter copper links is optional right?"

Honestly I don't know, without studying standards.  Personally, could see FEC being mandatory or optional.  A particular standard might make FEC mandatory if the "usual" (1E-10 to 12) expected bit error rates cannot otherwise be met.  (Understand, even FEC does not guarantee 100% no errors.)  Optional might be when a particular standard provides a way to better the "usual" expected bit error rates.

NB: just further skimmed a couple of your references, and indeed it appears FEC is only mandatory when the desired BER cannot otherwise be achieved.  "Trade-offs", on one of those references, better describes what I was trying to describe in my prior posting.

"And please let me make sure of "100GBASE-LR4" mentioned before, I think according to the above link,
Cisco QSFP-100G-LR4-S is not support for FEC right?"

Looks that way.  One of your references does seem to note whether FEC is part of the transceiver being used, or not.  But further confirmation, from likely more experienced engineers, using optics, might be found within the Optics forum.