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172 | Bidirectional Forwarding Detection
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Two important parameters are calculated using the values contained in the control packet.
• Transmit interval — Transmit interval is the agreed-upon rate at which a system sends control
packets. Each system has its own transmit interval, which is the greater of the last received remote
Desired TX Interval and the local Required Min RX Interval.
• Detection time — Detection time is the amount of time that a system does not receive a control
packet, after which the system determines that the session has failed. Each system has its own
detection time.
• In Asynchronous mode: Detection time is the remote Detection Multiplier multiplied by greater of
the remote Desired TX Interval and the local Required Min RX Interval.
• In Demand mode: Detection time is the local Detection Multiplier multiplied by the greater of the
local Desired Min TX and the remote Required Min RX Interval.
Table 9-1. BFD Packet Fields
Field Description
Diagnostic Code The reason that the last session failed.
State The current local session state. See BFD sessions.
Flag A bit that indicates packet function. If the poll bit is set, the receiving system must respond as
soon as possible, without regard to its transmit interval. The responding system clears the poll
bit and sets the final bit in its response. The poll and final bits are used during the handshake
and Demand mode (see BFD sessions).
Note: FTOS does not currently support multi-point sessions, Demand mode,
authentication, or control plane independence; these bits are always clear.
Detection Multiplier The number of packets that must be missed in order to declare a session down.
Length The entire length of the BFD packet.
My Discriminator A random number generated by the local system to identify the session.
Your Discriminator A random number generated by the remote system to identify the session. Discriminator
values are necessary to identify the session to which a control packet belongs since there can
be many sessions running on a single interface.
Desired Min TX Interval The minimum rate at which the local system would like to send control packets to the remote
system.
Required Min RX Interval The minimum rate at which the local system would like to receive control packets from the
remote system.
Required Min Echo RX The minimum rate at which the local system would like to receive echo packets.
Note: FTOS does not currently support the echo function.
Authentication Type
An optional method for authenticating control packets.
Note: FTOS does not currently support the BFD authentication function.
Authentication Length
Authentication Data