Bidirectional Forwarding Detection (BFD) is a simple Hello protocol that is similar to detection components of well-known routing protocols. A pair of systems transmit BFD packets periodically over each path between the two systems, and if a system stops receiving BFD packets for long enough, the neighboring system is assumed to have failed.
A BFD session is established based on the needs of the application that would use BFD. The user has to explicitly configure the address and parameters for the BFD session and the subscribers/applications (BGP/OSPF) of the session, as there is no discovery mechanism in BFD.
Routing protocols like BGP or OSPF exchange the learned routes between Edges and Routers. These protocols exchange routes and detect route failures using their own mechanism. Generally, route failures are detected based on the keepalive mechanism where one entity echoes other entity on a frequent configured interval, that is the keepalive time. These routing protocols have higher keepalive timers which results in longer duration to detect the route failures. BFD detects route failures between two connected entities faster with low overhead on detection of failures.
The following are the advantages of implementing BFD with routing protocols.
- Fast route failure detection with low re-convergence time.
- Less overhead in route failure detection.
- Uniform rate of route failure detection across routing protocols.
BFD can be defined as a simple service. The service primitives provided by BFD are to create, destroy, and modify a session, given the destination address and other parameters. BFD in return provides a signal to the clients indicating when the BFD session goes up or down.
There are two operating modes to BFD, asynchronous mode and demand mode. VMware supports asynchronous mode. In this mode, the systems periodically send BFD control packets to other systems and if several packets in a row are not received by a system, the session is declared to be down.
VMware supports BFD for the following routing protocols:
- BGP on Edges and Partner Gateways
- OSPF on Edges