Bidirectional Forwarding Detection

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.
The second mode is known as Demand mode. In this mode, it is assumed that a system has an independent way of verifying that it has connectivity to the other system. Once a BFD session is established, such a system may ask the other system to stop sending BFD Control packets, except when the system feels the need to verify connectivity explicitly, in which case a short sequence of BFD Control packets is exchanged. Demand mode may operate independently in each direction, or simultaneously.
An adjunct to both modes is the Echo function. When the Echo function is active, a stream of BFD Echo packet is transmitted in such a way as to have the other system loop them back through its forwarding path. If a number of packets of the echoed data stream are not received, the session is declared to be down. The Echo function may be used with either Asynchronous or Demand mode. Since the Echo function is handling the task of detection, the rate of periodic transmission of Control packets may be reduced (in the case of Asynchronous mode) or eliminated completely (in the case of Demand mode).

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