Dell PowerSwitch S4112F-ON OS10 Enterprise Edition User Guide Release 10.4.1.0 - Page 283
BFD session states
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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 for static and dynamic routing protocols, such as VRRP, OSPF, OSPFv3, IS-IS, and BGP. NOTE: In this release, BFD is only supported for BGP. 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. 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. Example: BFD session state changes 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 3 283
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