After the BFD session is established, the Palo Alto Networks
®
implementation
of BFD operates in asynchronous mode, meaning both endpoints send
each other control packets (which function like Hello packets) at
the negotiated interval. If a peer does not receive a control packet
within the detection time (calculated as the negotiated transmit
interval multiplied by a Detection Time Multiplier), the peer considers
the session down. (The firewall does not support demand mode, in
which control packets are sent only if necessary rather than periodically.)