Symptom
When connecting a Nexus 9500 N9K-X97160YC-EX linecard to a Catalyst 9000 switch while utilizing a SFP-10/25G-LR-S transceiver, FEC auto negotiations fails and the link does not come up. This is seen at 25Gbe speeds and does not impact 10G speed if statically set.
Conditions
This is seen when connecting inter-platform from a Nexus 9k N9k-X97160YC-EX linecard to a catalyst 9000. Furthermore, speed must be set to auto or statically set to 25Gbe.
Workaround
The port will come up if the speed is set to 10Gbe statically in the port configurations. This disables the need for FEC.
Further Problem Description
Problem is related to support of RS-FEC at 25Gbe in the Nexus platform. It is likely that this will apply to any device connecting to Nexus N9K-X97160YC-EX linecard except possibly another Nexus device.
Looking at output seen when the SFP-10/25G-LR-S transceiver is inserted in a Nexus 9000, we will see that that RS-FEC CL108 is not an option.
N9508(config-if)# fec ?
auto FEC auto
fc-fec CL74(25/50G)
off Turn FEC off
rs-fec CL91(100G) or Consortium 1.5 (25/50G)
