The following example
shows the ASA and a neighboring router running Border Gateway Protocol (BGP).
At the time when both devices come up, there is no BFD session established
Figure 1. Established BFD Session
After BGP identifies
its BGP neighbor, it bootstraps the BFD process with the IP address of the
neighbor. BFD does not discover its peers dynamically. It relies on the
configured routing protocols to tell it which IP addresses to use and which
peer relationships to form.
The BFD on the router
and the BFD on the ASA form a BFD control packet and start sending the packets
to each other at a one-second interval until the BFD session is established.
The initial control packets from either system are very similar, for example,
the Vers, Diag, H, D, P, and F bits are all set to zero, and the State is set
to Down. The My Discriminator field is set to a value that is unique on the
transmitting device. The Your Discriminator field is set to zero because the
BFD session has not yet been established. The TX and RX timers are set to the
values found in the configuration of the device.
After the remote BFD
device receives a BFD control packet during the session initiation phase, it
copies the value of the My Discriminator field into its own Your Discriminator
field and the transition from Down state to Init state and then eventually to
Up state occurs. Once both systems see their own Discriminators in each other's
control packets, the session is officially established.
illustration shows the established BFD connection.
Figure 2. BGP With No BFD Session Established