Users Guide

1396 Bidirectional Forwarding Detection
periodically and, if one stops receiving peer packets within the detection time
limit, it considers the bidirectional path to have failed. It then notifies the
application protocol of this failure.
BFD allows each device to estimate how quickly it can send and receive BFD
packets to agree with its neighbor upon how fast detection of failure may be
performed.
BFD operates between two devices on top of any underlying data protocol
(network layer, link layer, tunnels, etc.) as payload of any encapsulating
protocol appropriate for the transmission medium. The Dell EMC
Networking implementation of BFD works with IP networks (v4 and v6) and
supports IPv4/v6 address-based encapsulations.
BFD is standardized in RFC 5880.
BFD Operational Modes
BFD implements two main operational modes, as well as an additional
capability that may be used in combination with either of the two modes.
The two modes are Asynchronous mode and Demand mode, and the
additional capability is the Echo function.
Asynchronous Mode
This is the nominal operating mode for BFD. In this mode, the pair of devices
periodically sends BFD control packets to one another and, if a consecutive
number of those packets are not received by the other device, the session is
declared down.
The asynchronous mode is advantageous as it requires half the number of
packets to achieve a particular detection time as does to the echo function.
Demand Mode
In demand mode, it is assumed that a device has an independent way of
verifying that it has connectivity to the other system. Once a BFD session is
established, a demand mode device may ask the other to stop sending BFD
control packets, except when the device needs to verify the connectivity
explicitly. In this case, a short sequence of BFD Control packets, known as the
Poll Sequence, is exchanged to ascertain the connectivity. Demand mode may
operate independently in either direction.