BFD

The Poll Sequence:

A Poll Sequence is an exchange of BFD Control packets that is used in some circumstances to ensure that the remote system is aware of parameter changes. It is also used in Demand mode  to validate bidirectional connectivity.
A Poll Sequence consists of a system sending periodic BFD Control packets with the Poll (P) bit set. When the other system receives a Poll, it immediately transmits a BFD Control packet with the Final (F) bit set . When the system sending the Poll sequence receives a packet with Final, the Poll Sequence is terminated, and any subsequent BFD Control packets are sent with the Poll bit cleared. A BFD Control packet MUST NOT have both the Poll (P) and Final (F) bits set.