R2511-HP MSR Router Series High Availability Configuration Guide(V5)

• Echo packet mode—One end of the link sends Echo packets to the other end, which then forwards

the packets back to the originating end, monitoring link status in both directions.

BFD operating modes

Before a BFD session is established, BFD has two operating modes—active and passive.

• Active mode—BFD actively sends BFD control packets regardless of whether any BFD control

packet is received from the peer.

• Passive mode—BFD does not send control packets until a BFD control packet is received from the

peer.

At least one end must operate in active mode for a BFD session to be established.

After a BFD session is established, both ends must operate in one of the following BFD operating modes:

• Asynchronous mode—Both endpoints periodically send BFD control packets to each other. BFD

considers that the session is down if it receives no BFD control packets within a specific interval.

• Demand mode—No BFD control packets are exchanged after the session is established. It is

assumed that the endpoints have another way to verify connectivity to each other. However, either

host might still send BFD control packets if needed.

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 connectivity is found to be up, no more

BFD control packets are sent until the next command is issued.

NOTE:

• Only the asynchronous mode is supported.

• When a BFD session is maintained by sending Echo packets, the session is independent of the operatin

mode.

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.

Authentication modes

BFD provides the following authentication methods:

• Simple—Simple authentication.

• MD5—MD5 (Message Digest 5) authentication.

• SHA1—SHA1 (Secure Hash Algorithm 1) authentication.

BFD packet format

BFD control packets are encapsulated into UDP packets with port number 3784 for single-hop detection

or port number 4784 for multi-hop detection (also can be 3784 based on the configuration task). BFD

echo packets have a similar format as BFD control packets with UDP port number 3785 except that the

Desired Min TX Interval and Required Min RX Interval fields are null. Figure 3 illu

strates the packet format.