Thus, the software on the line cards must to work in primary and backup modes. Failure is usually detected using a heartbeat mechanism. Under one approach, when the system comes up, the primary line card starts sending heartbeat messages to a bus on the controller card after the after the primary line card has registered with the controller card. 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.