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Bi-directional Forwarding Detection
Bi-directional Forwarding Detection (BFD) is a light-weight, low-overhead, short-duration
detection of failures in the path between two systems. If a system stops receiving BFD messages
for a long enough period (based on configuration) it is assumed that a failure along the path has
occurred and the associated protocol or service is notified of the failure.
BFD can provide a mechanism used for liveness detection over any media, at any protocol layer,
with a wide range of detection times and overhead, to avoid a proliferation of different methods.
SR OS supports asynchronous mode of BFD in which BFD messages are set to test the path
between systems.
If multiple protocols are running between the same two BFD endpoints then only a single BFD
session is established, and all associated protocols will share the single BFD session.
In addition to the typical asynchronous mode, there is also an echo function defined within draft-
ietf-bfd-base-04.txt, Bi-directional Forwarding Detection, that allows either of the two systems to
send a sequence of BFD echo packets to the other system, which loops them back within that
system's forwarding plane. If a number of these echo packets are lost then the BFD session is
declared down.
BFD Control Packet
The base BFD specification does not specify the encapsulation type to be used for sending BFD
control packets. Instead it is left to the implementers to use the appropriate encapsulation type for
the medium and network. The encapsulation for BFD over IPv4 networks is specified in draft-ietf-
bfd-v4v6-1hop-04.txt, BFD for IPv4 (Single Hop). This specification requires that BFD control
packets be sent over UDP with a destination port number of 3784 and the source port number must
be within the range 49152 to 65535.
In addition, the TTL of all transmitted BFD packets must have an IP TTL of 255. All BFD packets
received must have an IP TTL of 255 if authentication is not enabled. If authentication is enabled,
the IP TTL should be 255 but can still be processed if it is not (assuming the packet passes the
enabled authentication mechanism).
If multiple BFD sessions exist between two nodes, the BFD discriminator is used to de-multiplex
the BFD control packet to the appropriate BFD session.
7450 ESS OS Router Configuration Guide
IP Router Configuration
Page 31
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