The secondary line card takes over from the primary line card when the controller card detects a long break in the heartbeat messages from the primary line card. If all messages come to the primary and secondary line cards, the secondary card can build the same set of tables that the primary card possesses. Some vendors use a system wherein the physical line is attached to both line cards. An external switch blocks all transmit from the standby port, but the receive function on both ports is always enabled. So the active and standby ports are in lockstep as they receive and transmit status and status inquiry messages.
Data traffic is ignored on the standby.
As an alternative to the centralized controller switchover method, the active and standby can exchange keep-alive messages directly. If there is no response after a maximum number of retries, the standby takes over by sending a toggle command to the external switch. A certain amount of data buffering at the standby could be used as the rollback for the last time a keep-alive was received. The standby now processes both the control and data messages. If the failed active board is restored, it takes over as the new standby board.