25G AOC between Nexus 93180YC-FX and Arista 7050SX3 stays notconnect until FEC is set to rs-ieee
2
The poster was connecting a Cisco Nexus 93180YC-FX to an Arista 7050SX3 with a 25G AOC sold as Cisco/Arista compatible. The Nexus detects the transceiver, but 'show interface status' reports the port as not connected. A 10G AOC from the same vendor works fine between the two switches, and the same 25G AOC links at 25G when looped back-to-back on the Nexus (and, per the poster, also back-to-back on the Arista). Duplex, speed and auto-negotiation settings were changed without effect, and the poster asked whether this was a Nexus driver problem or a Cisco/Arista compatibility issue.
- Hardware: Cisco Nexus 93180YC-FX, Arista 7050SX3
- Transceivers: third-party 25G SFP28 AOC (Cisco/Arista compatible), third-party 10G SFP+ AOC
- Form factor: SFP28, AOC, SFP+
Symptoms / messages:
transceiver detected but show interface status shows notconnect
25G AOC link down between Nexus 93180YC-FX and Arista 7050SX3
same AOC links at 25G back-to-back on either vendor
10G AOC from the same vendor works
Comments 3
Accepted solution: 25G links generally require matching FEC on both ends. Nexus 9000 switches auto-select a FEC mode from the cable's PID (e.g. SFP-25G-AOC3M or SFP-25G-SR-S) and the Arista may pick a different mode, which explains why the cable works back-to-back on one platform but not across vendors. Hard-code FEC on both sides, on NX-OS with the interface command 'fec rs-ieee' and the equivalent setting on Arista EOS.
The poster confirmed it was a FEC mismatch: after configuring 'fec rs-ieee' the 25G link came up.
A respondent asked for interface output and device logs and suggested repeating the back-to-back test on the Arista side, since nothing so far pointed at the Nexus.