BFD operating modes
BFD can operate in any of three available modes: asynchronous, demand, and echo. The three BFD modes are described in the following paragraphs.
- Asynchronous mode—In this mode, the systems periodically send BFD control packets to one another, and if a number of those packets in a row are not received by the other system in a stipulated time, the session is declared to be down.
- Demand mode—In this mode, a system has an independent way of verifying that it has connectivity to the other system. After a BFD session is established, such a system may ask the other system to stop sending BFD control packets, except when the system detects the need to verify connectivity explicitly. In such a case, a short sequence of BFD control packets is exchanged. Demand mode may operate independently in each direction or simultaneously.
- Echo mode—In this mode, packets are transmitted and consumed by the originating system. This mode exercises the forwarding path of the remote end. Echo mode is used when more-aggressive intervals are required.