BFD session establishment

BFD does not provide any neighbor discovery mechanisms. The upper protocol notifies BFD of the routers to which it needs to establish sessions.

A BFD session is established as follows:

1. A protocol sends Hello messages to discover neighbors and establish neighborships.

2. After establishing a neighborship, the protocol notifies BFD of the neighbor information, including destination and source addresses.

3. BFD uses the information to establish a BFD session.

When BFD detects a link failure, it performs the following tasks:

1. BFD clears the neighbor session.

2. BFD notifies the protocol of the failure.

3. The protocol terminates the neighborship on the link.

4. If a backup link is available, the protocol will use it for communication.