After we upgrade servers to 2.15.5RC2, clients were in RC1 can't mount with following order:
mount -t lustre 172.16.5.5@tcp,172.16.5.6@tcp:172.16.5.7@tcp,172.16.5.8@tcp:172.16.5.3@tcp,172.16.5.4@tcp:172.16.5.1@tcp,172.16.5.2@tcp:/testfs /lustre/testfs/client

However, this order worked
mount -t lustre 172.16.5.1@tcp,172.16.5.2@tcp:172.16.5.3@tcp,172.16.5.4@tcp:172.16.5.5@tcp,172.16.5.6@tcp:172.16.5.7@tcp,172.16.5.8@tcp:/testfs /lustre/testfs/client

for the clients that update to RC2, mount worked with both order