34.1.3 BFD session-session

A BFD session is created between the Switch and the partner switch by setting the partner switch as the target of monitoring by BFD. In this context, a target is a connection target that is uniquely determined by the following combinations:

Destination IP address
Transmit interface for BFD packets. (Single-hop only)

If a BFD is not set on the Switch, no BFD is established because the Switch does not respond to BFD packets received from the partner switch. To generate a BFD session. the Switch and the partner switch must be configured in both directions.

<Structure of this section>

(1) Basic operation

BFD of the Switch operates based on the requirements of the protocols to be linked. When BFD is set to the configuration as a protocol-monitoring function and the partner switch to be monitored is determined, new BFD sessioncreation and establishment starts. The method of determining the partner switch depends on the protocol. If the configuration is changed, it is reflected in the operation. Also, if the configuration is deleted, BFD sessionwill be deleted.

The following diagram shows the basic operation of BFD.

Figure 34-1: Basic operation of BFD

On Switch A, set Switch C and Switch D as BFD monitoring targets. Switch C sets Switch A as BFD monitoring target for Switch A and Switch D. At this time, BFD sessions are generated between Switch A and Switch C, Switch A and Switch D, respectively.

(2) BFD Session Status

The generated BFD session starts monitoring according to the status. The following tables show BFD session-status.

Table 34-2 BFD Session Status
Status	Description	Description
Down	Down	This is the initial status of the generated BFD session. Or, the partner switch is not recognized.
Init	Establishment request in progress	This is a transient status in which Down status BFD packets are received from the partner switch and only the local switch recognizes the monitoring target. It may not go through Init status and go into the direct Up status.
Up	Up	A Init or Up status BFD datagram is received, and the monitored objects are recognized by each other. The session is established and BFD monitoring is enabled.
AdminDown	Administrative downtime	BFD session has been intentionally brought down by the user or by thesystem. Establishing a BFD session is suppressed, and BFD session goes down when the partner switch notifies of AdminDown status.

BFD datagram contains the status of the sending device. The status of BFD session is determined by the status on BFD packet. The status of the receiving device is determined.

The following diagram shows the status transitions for BFD sessions between Switch A and Switch C.

Figure 34-2: BFD state transitions

Switches A and C send BFD packets (Down) to BFD monitored system at regular intervals by creating BFD sessions in Down according to the configuration.
When Switch C receives a BFD packet (Down), it transitions to Init status and sends BFD packet (Init) at regular intervals because it recognizes Switch A. Upon receiving BFD packet. (Init), Switch A enters Up status.
Switch A sends BFD packets (Up) at regular intervals. Switch C enters Up status when it receives a BFD packet. (Up) Init status.

In this way, both units are considered to be in Up status to establish a BFD session. This enables BFD monitoring.

If one of the switches is in Down or AdminDown status during an established session, the partner switch is also in Down status due to the reception of BFD packetsand BFD session is down.