When upgrading from Lustre 1.8 to 2.1, if the parameter rootsquash is set, Lustre throws an error and denies mounting the filesystem. This is the error from the MDS logs:

Mar 7 20:03:35 mds01 kernel: LustreError: 6610:0:(obd_config.c:1140:class_process_proc_param()) home-MDT0000: unknown param rootsquash=99:99
Mar 7 20:03:35 mds01 kernel: LustreError: 6610:0:(obd_config.c:1361:class_config_llog_handler()) Err -38 on cfg command:
Mar 7 20:03:35 mds01 kernel: Lustre: cmd=cf00f 0:home-MDT0000 1:mdt.rootsquash=99:99
Mar 7 20:03:35 mds01 kernel: LustreError: 15c-8: MGC172.16.4.3 at tcp: The configuration from log 'home-MDT0000' failed (-38). This may be the result of communication errors between this node and the MGS, a bad configuration, or other errors. See the syslog for more information.
Mar 7 20:03:35 mds01 kernel: LustreError: 6124:0:(obd_mount.c:1192:server_start_targets()) failed to start server home-MDT0000: -38
Mar 7 20:03:35 mds01 kernel: LustreError: 6124:0:(obd_mount.c:1723:server_fill_super()) Unable to start targets: -38
Mar 7 20:03:35 mds01 kernel: Lustre: Failing over home-MDT0000

We were able to overcome this using tunefs.lustre with the option --writeconf added. This should be managed automatically (ideally, migrating the configuration) or at least, documented.