34.1.4 Fault detected by BFD

<Structure of this section>

(1) How Fault Detection Works

After a BFD session is established, if no BFD packets are received within the fault-detection period, BFD session is down.

The following figure shows a failure between Switch D and HUB.

Figure 34-3: Failures in BFD Sessions

Because there is a HUB between Switch A and Switch D, Switch A cannot detect a link failure and remains linked up. However, by not receiving BFD packets, BFD session can detect a failure after the failure-detection period has elapsed. In this way, BFD is useful when there are L2 switches between systems, for example, when a failure of the partner switch is not communicated.

(2) Setting the Fault Detection Time

The failure detection time is determined by the monitoring interval set for the Switch in the configuration and the monitoring interval set for the partner switch. The following table describes how to set each monitoring interval and calculate the failure detection time for the Switch.

Table 34-3: Setting and calculating monitoring intervals
Decision maker	Monitoring interval	Description
Switch	Minimum transmission interval	This is the minimum interval for sending BFD packets requested by the Switch. This parameter is used to calculate the amount of time between BFD packets that the Switch sends to the partner switch. Set in the configuration If you set a value less than one second, it is one second before the session is established.
	Minimum reception interval	This is the minimum interval for receiving BFD packets requested by the Switch. This parameter is used to calculate the amount of time between BFD packets sent by the partner switch to the Switch. Set in the configuration
	Detection multiplier	This is the detection multiplier requested by the Switch. Indicates the number of consecutive packet loss to be handled as a failure. Set in the configuration
Facing device	Minimum transmission interval	This is the minimum transmission interval requested by the partner switch. The Switch is notified of BFD sent by the partner switch.
	Minimum reception interval	This is the minimum reception interval requested by the partner switch. The Switch is notified of BFD sent by the partner switch.
	Detection multiplier	This is the detection multiplier requested by the partner switch. The Switch is notified of BFD sent by the partner switch.
Switch	Sending interval	This is BFD sending interval for the Switch. Compare the minimum transmission interval of the Switch with the minimum reception interval of the opposing devices, and adopt the larger value.
	Reception interval	This is the interval for receiving BFD packets from the Switch. Compare the minimum reception interval of the Switch with the minimum transmission interval of the opposing devices, and adopt the larger value.
	Fault detection time	If a BFD is not received within the failure detection time, it is regarded as a failure and BFD session is brought down. This is the value obtained by multiplying the reception interval of the Switch by the detection multiplier requested by the partner switch.

The transmit and receive intervals do not have to be the same value. The packet transmission/reception interval is determined independently for each communication direction.

The failure detection time of the Switch must not exceed 300 seconds.

(3) Fault detection operation

The following figure shows an example of detecting a fault during a BFD session. The following shows an example from the perspective of the Switch: The detection multiplier is 3.

Figure 34-4: Detecting Faults in BFD Sessions

When a failure is detected, BFD session is notified to the associated protocol. The transmission of BFD packets is stopped. For protocols that dynamically determine a target, the protocol removes BFD session. The target is then re-elected.