3Com S7902E, Configuration Manual

Page: 2243 / 2621

1-4
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
Figure 1-3 illustrates the BFD control packet format. BFD control packets are encapsulated into UDP
packets with port number 3784.
Figure 1-3 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 protocol from up to some other
state. Table 1-1 lists the states.
Table 1-1 Diag bit values
Diag Description
0 No Diagnostic
1 Control Detection Time Expired
2 Echo Function Failed
3 Neighbor Signaled Session Down
4 Forwarding Plane Reset
5 Path Down
6 Concatenated Path Down