OS10 Enterprise Edition User Guide Release 10.4.1.0

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BFD three-way handshake

A BFD session requires a three-way handshake between neighboring routers. In the following example, the handshake assumes:
  • One router is active, and the other router is passive.
  • This is the first session established on this link.
  • The default session state on both ports is Down.
  1. The active system sends a steady stream of control packets to indicate that its session state is Down, until the passive system responds. These packets are sent at the desired transmit interval of the Active system. The Your Discriminator field is set to zero.
  2. When the passive system receives a control packet, it changes its session state to Init and sends a response to indicate its state change. The response includes its session ID in the My Discriminator field and the session ID of the remote system in the Your Discriminator field.
  3. The active system receives the response from the passive system and changes its session state to Up. It then sends a control packet to indicate this state change. Discriminator values are exchanged, and transmit intervals are negotiated.
  4. The passive system receives the control packet and changes its state to Up. Both systems agree that a session is established. However, because both members must send a control packet, which requires a response, whenever there is a state change or change in a session parameter, the passive system sends a final response indicating the state change. After this, periodic control packets are exchanged.
BFD three-way handshake

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