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BFD Overview
This implementation of BFD associates Echo packets in UDP packets using port 3785 and the IP address
of the transmitting interface. The contents of the Echo packet is defined as follows:
Field
Version
My Discriminator
Sequence Number
BFD Session Establishment
There are three states through which a BFD session normally proceeds: two for establishing a session (Up
and Init state) and one for tearing down a session (Down state). In addition, an AdminDown state exists to
administratively take down a session.
BFD uses a three-way handshake to establish sessions and guarantee that each BFD peer is aware of all the
state changes. The transmitting system fills the state field in the transmitted BFD control packet with its
current session state. To establish a session, the receiving peer system changes its session state based on
the state field value in the received BFD control packet and its own session status.
A Down state means that a session is down or has been recently created. A session remains down until the
remote system sends a packet with any state other than an up state. If a BFD packet with the state field set
to down is received by the local system that is also in a down state, the session advances to Init state; if
that packet signals Init state, the session advances to Up state.
Init signals that there is communication between the systems and that the local system wishes to start a
session but the remote system has not yet acknowledged it. The session stays at Init until the local system
receives a control packet with Init or Up in its state field (in which case the session state moves to Up) or
until the detection time limit is reached.(in which case the remote system is then considered unreachable
and the state moves to Down)
An Up state indicates that a BFD session has been created and both BFD peers are communicating with
each other. The BFD session continues 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 are 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 (converted back
into their original separate signals).
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
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Description
The version number of the BFD
protocol.
An identifier for the BFD session
connecting to the local side.
The sequence number for this
packet. This value is
incremented for each successive
packet transmitted for a session.
OmniSwitch AOS Release 7 Network Configuration Guide
Configuring BFD
March 2011
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