H3C Technologies H3C S7500E Series Switches - Manual (Page 176)

11-4
z At present, only the asynchronous mode is supported.
z When a BFD session operates in Echo mode, the session is independent of the operating
mode.
z When the connectivity to another system needs to be verified explicitly, a system sends
several BFD control packets that have the Poll (P) bit set at the negotiated transmit interval.
If no response is received within the detection interval, the session is considered down. If
the connectivity is found to be up, no more BFD control packets are sent until the next
command is issued.
Dynamic BFD parameter changes
After a BFD session is established, both ends can negotiate the related BFD parameters, such
as the minimum transmit interval, minimum receive interval, initialization mode, and packet
authentication mode. After that, both ends use the negotiated parameters, without affecting the
current session state.
BFD Packet Format
BFD control packets are encapsulated into UDP packets with port number 3784. BFD echo
packets have a similar format as BFD control packets (except that the Desired Min TX Interval
and Required Min RX Interval fields are null) with UDP port number 3785. Figure 11-3
illustrates the packet format.
Figure 11-3 BFD control packet format
z Vers: Protocol version. The protocol version is 1.
z Diag: This bit indicates the reason for the last transition of the local session from up to
some other state. Table 11-1 lists the states.
Table 11-1 Diag bit values
Diag Description
0 No Diagnostic