On 21.4.L4 version Juju Contrail Agent Upgrade may not be successful if there are multiple kernels present on the Compute
We may see error messages like this in Compute's vrouter kernel init logs.
+ echo 'INFO: vrouter.ko is NOT built for current kernel'
INFO: vrouter.ko is NOT built for current kernel
ERROR: "del_timer_sync" [/usr/src/vrouter-21.4.L4.76/vrouter.ko] undefined!
+ echo 'Kernel 5.4.0-204-generic is unsupported. Module has not been compiled.'
This issue is a bug and is fixed in 21.4.L4.76 onwards (contrail-vrouter-kernel-build-init:21.4.L4.76 container has the fix).