Users Guide

BFD is a simple hello mechanism. Two neighboring routers running BFD establish a session using a three-way handshake. After
the session is established, the routers exchange periodic control packets at sub-second intervals. If a router does not receive a
hello packet within the specified time, routing protocols are notified that the forwarding path is down.
In addition, BFD sends a control packet when there is a state change or change in a session parameter. These control packets
are sent without regard to transmit and receive intervals in a routing protocol.
BFD is an independent and generic protocol, which all media, topologies, and routing protocols can support using any
encapsulation. OS10 implements BFD at Layer 3 (L3) and with User Datagram Protocol (UDP) encapsulation. BFD is supported
on static and dynamic routing protocols, such as static route, OSPF, OSPFv3, and BGP.
The system displays BFD state change notifications.
NOTE: OS10 does not support BFD sessions on VLT port channel interfaces. BFD sessions on other interfaces (non-VLT
links) are supported on VLT peers.
BFD session states
To establish a BFD session between two routers, enable BFD on both sides of the link. BFD routers can operate in both active
and passive roles.
● The active router starts the BFD session. Both routers can be active in the same session.
● The passive router does not start a session. It only responds to a request for session initialization from the active router.
A BFD session can occur in Asynchronous and Demand modes. However, OS10 BFD supports only Asynchronous mode.
● In Asynchronous mode, both systems send periodic control messages at a specified interval to indicate that their session
status is Up.
● In Demand mode, if one router requests Demand mode, the other router stops sending periodic control packets; it only sends
a response to status inquiries from the Demand mode initiator. Either peer router, but not both, can request Demand mode
at any time.
A BFD session can have four states: Administratively Down, Down, Init, and Up. The default BFD session state is Down.
● Administratively Down — The local BFD router does not participate in the session.
● Down — The remote BFD router is not sending control packets or does not send them within the detection time for the
session.
● Init — The local BFD outer is communicating to the remote router in the session.
● Up — Both BFD routers are sending control packets.
A BFD session's state changes to Down if:
● A control packet is not received within the detection time.
● Demand mode is active and a control packet is not received in response to a poll packet.
BFD session state changes example
The session state on a router changes according to the status notification it receives from the peer router. For example, if the
current session state is Down and the router receives a Down status notification from the remote router, the session state on
the local router changes to Init.
Layer 3
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