Good day,
Faced very interesting problem connecting ASR9901 with Catalyst 9500 48Y4C over 100G LR4 optics.
Links between two 9901's or two 48Y4C's are raising fine, meanwhile link between any router and any switch is dead.
Troubleshooting different config options have found, that disabling FEC on switch brings interface UP on both ends.
Continued to investigate and forcefully enabled FEC on all devices and 100G interfaces (on routers "normal" on switches "clause91" both coresponding to Reed-Solomon), and again links between routers and switches are down.
Routers are running Cisco IOS XR Software, Version 6.5.2
Catalysts are on Version 16.9.4
Respective config parts are below
Catalyst
interface HundredGigE1/0/50
description ASR9901-0/0/0/20
no ip address
fec off
!
ASR
interface HundredGigE0/0/0/20
description 9500-1/0/50
!
controller preconfigure HundredGigECtrlr0/0/0/20
fec standard (removing or putting this does no effect on link state)
!
