BFD Overview Configuring BFD
page 30-12 OmniSwitch AOS Release 6 Network Configuration Guide September 2009
An Up state indicates that a BFD session has been created and both BFD peers are communicating with
each other. The BFD session will continue to remain in this state until connectivity fails and the state
moves to Down or until the BFD session is taken down administratively.
Demultiplexing
Each BFD session must be able to uniquely identify itself and received BFD packets among the myriad of
BFD sessions that may be running. Each BFD peer must choose an identifying and unique discriminator
value. This value is sent in the “My Discriminator” field of the BFD control packet, and is reflected back
in the “Your Discriminator” field of the control packet sent from the remote peer. Once the system has
echoed the respective “Your Discriminator” value back to its peer, the packets are demultiplexed (i.e.,
converted back into their original separate signals). The source address and interfaces may change but will
continue to be associated with the proper session.
BFD Timer Negotiation
The BFD control packet contains information about how quickly a system would like to send packets to its
peer, as well as how rapidly it is willing to receive packets from the peer. The BFD detection time is not
carried explicitly in the protocol, but rather, it is determined by the receiving system independently based
on the transmission interval (TX) and Detection Multiplier that have been negotiated.
The Detection Multiplier field value is approximately the number of packets that must be missed in order
to declare a session down. In Asynchronous mode, detection times can be different in each direction. The
local system detection time in this mode equals the value of Detection Multiplier received from the remote
system multiplied by the negotiated transmission interval (TX). Because the time values for BFD control
packet transmissions and session detection are being constantly negotiated by the participating BFD peers,
they can be changed at any time. They are also independent in each direction for each session.
To change the rate at which BFD control packets are received, you can change the Required Min RX Inter-
val at any time to any value. This new value will be sent in the next outgoing packet so that the remote
system can accommodate the changes made. Similarly, to change the rate at which BFD control packets
are transmitted, you can change the Desired Min TX Interval at any time to any value.
With some exceptions, a system cannot transmit control packets with an interval shorter than the larger
value of the TX interval and RX interval fields. This means that the system with the slower rate deter-
mines the BFD control packet transmission speed.
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