Bidirectional Forwarding Detection (BFD)

BFD (Bidirectional Forwarding Detection) is basically another Hello protocol based on UDP that detects link failures on forwarding plane. BFD runs on top of Layer 2 protocols that is in use between two adjacent systems.

BFD mechanism is independent from media, routing protocols and data protocols. By being independant from media, routing protocols and data protocols, BFD mechanism can be used with all routing protocols and data protocols. BFD mechanism is over these protocols.

Routing protocols has an hello mechanism by default to detect the link failures. But this mechanism is differenciate from router to router. With BFD mechanims, a common link failure detection mechanism created. And this BFD mechanism can be used with all routing protocols. Using BFD mechanism provides a very fast convergence after a link failure if you compare with routing protocol hello mechanism.

BFD (Bidirectional Forwarding Detection) is one way protocol. So tu useBFD mechanims, you need to configure both ends.

BFD operates on both direct links or multi hop MPLS LSPs.

As you know there are two planes in a network. One of them is control plane, in which, route processing is done. And the other is forwarding plane, in which, the data sending is done after the establishment of the session.

BFD mecahnism operates in the forwarding plane. It detects the link failures on forwarding plane.

BFD has two versions. These are Version 0 and Version 1.

BFD Session Establishment

Firtly BFD Session Establishment must be done to use BFD mechanism. This is done through BFD messages. These messages set two ends in three different states as mentioned below:

• Down
• Init
• Up

BFD mechanims has two different BFD Session Establishment modes. This is divided into two according to how is the discriminators that differ BFD sessions are configured. What are these BFD Session Establishement modes? These are :

• Static Session Establishment
• Dynamic Session Establishment

In Static Session Establishment, local and remote discriminators in BFD messages are configured with commands. In Dynamic Session Establishment, this is done by system dynamically.

BFD Session Establishment is the first step of BFD Mecahnims. Let’ s check BFD Session Establishment Steps one by one.

As you can below, BFD Session Establishment is done with a three-way handshake. There are three basic steps to establish BFD Session in other words.

1) Firstly, the two ends that will use BFD are in down state. This is first state of both ends.

2) When a BFD message comes from one end to the other end as Down State, the State field is changed as Init in that end. And this is the second state.

3) After being in the Init state, the message with Init satte is sent and when the other end receive it, it changes state filed as UP state. This is the last and third state.