Bi-Directional Forwarding Detection; Bfd Control Packet - Alcatel-Lucent 7210 SAS M Configuration Manual

Bi-directional Forwarding Detection

Bi-directional Forwarding Detection (BFD) is a light-weight, low-overhead, short-duration
mechanism to detect 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.
Listed below are the advantages of implementing the BFD mechanism:
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BFD is implemented in asynchronous mode, in this mode periodic BFD control messages are used
to test the path between the systems.
A path is declared operational when two-way communication has been established between both
the systems. A separate BFD session is created for each communication path and data protocol
between two systems.
BFD also supports the Echo function defined in draft-ietfbfd-base-04.txt, Bidirectional
Forwarding Detection. In this scenario one of the systems send a sequence of BFD echo packets to
the other system which loops back the echo packets within the systems forwarding plane. If many
of the echo packets are lost, the BFD session is declared as down.

BFD Control Packet

The base BFD specification does not specify the encapsulation type to be used for sending BFD
control packets. Choice of the appropriate encapsulation-type to be implemented is based on the
network and medium. 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.
Note:
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7210 SAS M, X Router Configuration Guide
Used for Liveness detection over any media type
Can be used at any protocol layer
Proliferation of different methods and be avoided.
Can be used with a wide range of detection times and overhead
The TTL of all transmitted BFD packets must have an IP TTL of 255
If authentication is not enabled, all BFD packets received must have an IP TTL of 255.
If authentication is enabled, the IP TTL should be 255. In case the IP TTL is not 255 the
BFD packets are still processed, if packet passes the enabled authentication mechanism.
IP Router Configuration
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