25G connection between Nexus 93180YC-FX to Arista 7050SX3

[UpenT.]

Hi,

I am trying to connect 25G AOC (cisco/arista compatible). The interface can detect the transceiver but on show int eth status it show not connected. The same vendor 10G AOC works perfectly fine.Also if I connect same 25G AOC back to back on Nexus device  the interface comes up with 25G speed. Is this because of cisco Nexus driver issue or anything relating to compatibility with Arista?  But if there was issue with compatibility then 10G shouldn't work. Any information is highly appreciated.

Things I tried:

1. Duplex Settings

2. Speed setting

3. Auto Negotiation

Thanks in advance.

[balaji.bandi]

You need to provide interface output also what Logs you see on the device, since we do not any visibility what tested and what was the logs were generated at the time of testing.

The same vendor 10G AOC works perfectly fine.Also if I connect same 25G AOC back to back on Nexus device  the interface comes up with 25G speed.

as pre this information we do not see any issue on nexus side, have you done the same test on arista side ?

 But if there was issue with compatibility then 10G shouldn't work.  - you need to provide more information as I mentioned earlier.

[UpenT.]

Hi,

Really appreciate the response, I've done the same test on Arista side as well (back to back) works there too. Regarding logs and outputs will share is ASAP.

Thanks Again

[Christopher Hart · Accepted solution]

Hello!

With most 25Gbps connections, you will need to take FEC (Forward Error Correction) settings into account in order to bring a 25Gbps link up.

When working with fixed-length cables (such as the AOC cables you are working with), Cisco Nexus 9000 series switches will auto-select a recommended FEC settings based upon the PID (Product Identifier - for example, SFP-25G-AOC3M or SFP-25G-SR-S) of the inserted cable/transceiver. It is possible that the FEC selected by the Arista 7050SX3 is different than the FEC selected by the Nexus 93180YC-FX switch. Similarly to speed or duplex settings, the FEC setting must match on both sides of a link in order to successfully bring the link up. This would explain why connecting both ends of the same cable to the Nexus results in the link coming up, but connecting the cable to the Arista results in the link staying down.

I would recommend hard-coding the FEC configuration on both sides of the link (most likely to the IEEE's implementation of RS-FEC). The interface command to do this on NX-OS is fec rs-ieee - the command to do this on Arista's EOS is likely very similar, but consult Arista's documentation for more information.

I hope this helps - thank you!

-Christopher

[UpenT.]

Hi Christopher,

Thank you very much for you reply will definitely give this a try and update.

REALLY APPRECIATE YOUR TIME FOR REPLY.

[UpenT.]

Hi Christopher,

Thankyou very much for your post, it was indeed fec issue.Once I initiate fec rs-ieee command the link came up. Really appreciate it.

~UT

[Christopher Hart]

Hello!

Excellent! I'm happy to hear we were able to get to the bottom of the issue.

Thank you!

-Christopher