Business critical application demands convergence very fast but on shared IP infrastructure, it’s difficult to meet the requirement. Even with redundant network, in case of failure of device or link how fast the convergence can happen? May be in seconds or minutes. BFD(Bi Directional Forwarding Detection) is a protocol which is used to achieve the convergence of links in sub seconds. The fastest convergence could be achieved by IGP also which is almost equal to one minute not less than that but by using BFD it could be in sub seconds like we get in SDH/SONET. BFD is a protocol which uses hello packets to exchange the information of neighbor with irrespective of the protocol used (It can work with OSPF, EIGRP, BGP, LDP, ISIS. PWE3 etc.). The Internet Draft for BFD does not specify a specific encapsulation type for BFD control packets; rather, it recommends the use of an encapsulation “appropriate to the medium and the network”. BFD payload control packets will be encapsulated in UDP packets, using destination port 3784 and a source port in the range of 49152 to 65535.
The primary mode is known as Asynchronous mode. In this mode, the systems periodically send BFD Control packets to one another, and if a number of those packets in a row are not received by the other system, the session is declared to be down.