Bfd Control Packets; Bfd Echo Packets; Bfd Session Establishment - Alcatel-Lucent OmniSwitch 6850-48 Network Configuration Manual

Configuring BFD

BFD Control Packets

There is no specific encapsulation type for BFD control packets; instead, the BFD Internet Draft recom-
mends an encapsulation type that is "appropriate to the medium and network" used. This implementation
of BFD for IPv4 routing protocols (BGP, OSPF, VRRP Remote Tracking, and static routes), encapsulates
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. Authentica-
tion 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 encapsulates 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 will stay 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)
OmniSwitch AOS Release 6 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 incre-
mented for each successive
packet transmitted for a session.
September 2009
BFD Overview
page 30-11