Bfd Session Establishment - Alcatel-Lucent OmniSwitch 9900 Series Network Configuration Manual

Configuring BFD
implementation of BFD for IPv4 routing protocols (BGP, OSPF, VRRP Remote Tracking, and static
routes), associates BFD control packets in UDP packets using destination port 3784 and a source port in
the range of 49152 to 65535.
Note. The BFD control packet has a mandatory section and an optional authentication section.
Authentication is not supported in this implementation of the BFD protocol.
BFD Echo Packets
There is no specific definition for Echo packet format. The only requirement is that the transmitting
system is able to use the packet contents to distinguish between the various BFD sessions so that packets
are correctly processed for the appropriate session.
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.
OmniSwitch AOS Release 8 Network Configuration Guide
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.
December 2017
BFD Overview
page 20-12