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Configuring BFD
Operational Mode and Echo Function
The BFD protocol offers two different modes of operation:
•
Asynchronous mode
•
Demand mode (not supported)
This implementation of BFD supports the Asynchronous mode. In this mode, BFD neighbors periodically
send BFD control packets to each other. A time interval for transmitting and receiving such packets is
negotiated between the two BFD systems. If a neighboring system fails to receive a number of control
packets continuously over a specific period of time, the session is considered down and BFD informs the
appropriate routing protocol.
In addition to the operational mode, an Echo function is available to verify the forwarding path between
neighboring BFD systems. When enabled, a BFD system transmits Echo packets to a BFD neighbor,
which then sends the packets back to the originating system along the forwarding path. If no Echo packets
are received back from the BFD neighbor within a configured Echo time interval, the session is
considered down.
The Echo function is a configurable option and can work on its own or simultaneously with the
Asynchronous mode. Note that using the Echo function with the Asynchronous mode lowers the rate at
which control packets are sent because Echo packets are then used to detect session liveliness. In addi-
tion, transmitting Echo packets is only allowed over a single hop; transmitting BFD control packets is
allowed over multiple hops.
Once a BFD session is started, the BFD peers can decide whether or not Echo packets are actually trans-
mitted. A session is considered down when the peers receive no BFD control packets from each other or if
sufficient Echo packets are missed within a specific period of time.
BFD Packet Formats
The detection packets BFD sends are UDP packets which are of two types: BFD control packets and Echo
packets.
BFD Control Packets
There is no specific encapsulation type for BFD control packets; instead, the BFD IETF RFC-5880
recommends 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), 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.
OmniSwitch AOS Release 7 Network Configuration Guide
March 2011
BFD Overview
page 16-9
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