Control messages originated by the CPM that pertain to NAT redundancy are treated with the highest priority and are marked by the system with a DSCP value of Network Control NC1 (48d). Those messages are crucial to stability of NAT redundancy (otherwise inadvertent switchovers could occur).
Flow synchronization control messages originated by ISAs are marked with EF (46d) for DSCP and 5 for dot1p, which are lower priorities than CPM-originated messages. The sync (ISA keepalive) messages are sent with DSCP NC1 and dot1p 6.
Flow synchronization messages are more tolerant to delays and loss.
Both of these types of control messages should be higher priority than any customer -originated traffic that is expected to cross the ICL. Customer traffic can be marked in SR OS node by the appropriate QoS configuration on egress interfaces.