BFD in Demand Mode over a Point-to-Point MPLS LSP

Use of the BFD Demand Mode defines that the Demand mode may be:

asymmetric, i.e. used in one direction of a BFD session;
switched to and from without bringing BFD session to Down state through using a Poll Sequence.

For the case of BFD over MPLS LSP, ingress Label switching Edge Router (LER) usually acts as Active BFD peer and egress LER acts as Passive BFD peer. The Active peer bootstraps the BFD session by using LSP ping. If the BFD session is configured to use the Demand mode, once the BFD session is in Up state the ingress LER switches to the Demand mode as defined in Section 6.6 . The egress LER also follows procedures defined in Section 6.6 and ceases further transmission of periodic BFD control packets to the ingress LER. In this state BFD peers remain as long as the egress LER is in Up state. The ingress LER can periodically check continuity of a bidirectional path between the ingress and egress LERs by using the Poll Sequence, as described in Section 6.6 . An implementation that supports using the Poll Sequence as the mechanism for bidirectional path continuity check must control the interval between consecutive Poll Sequences. The default value could be selected as 1 second. If the Detection Time at the egress LER expires, the session state on the egress LER transitions to the Down state according to Section 6.2 of . As a result, the BFD system exits the Demand mode and sends the BFD Control packet to the ingress LER with the Status (Sta) field set to the Down (1) value and the Diagnostic (Diag) field set to Control Detection Time Expired (1) value. The egress LER periodically transmits these Control packets to the ingress LER until either it receives the valid BFD control packet from the ingress LER. The interval between BFD control packets is determined according to Section 6.8.7 of . The ingress LER transmits BFD Control packets over the MPLS LSP with the Demand (D) flag set at the negotiated interval per . If it receives the valid BFD packet from the egress LER with the Status (Sta) field set to Down and the Diagnostic (Diag) field value Control Detection Time Expired, the ingress LER indicates that the monitored LSP has a failure. Consequently, the BFD system on the ingress LER advances the state of the BFD session to the Init state and sends a BFD control packet with the Demand (D) flag cleared. That packet is transmitted periodically. The BFD system continues performing in the BFD Asynchronous mode until the BFD session reaches the Up state. Then the ingress LER switches to the Demand mode.

The Applicability of BFD for Multipoint Networks and define the use of BFD in multipoint networks. This specification analyzes the case of p2p LSP. In that scenario, the ingress of the LSP acts as the MultipointHead, and the egress - as MultipointTail. The BFD state machines for MultipointHead, MultipointClient, and MultipointTail don't use the three-way handshakes for session establishment and teardown. As a result, the Init state is absent, and the session transitions to the Up state once the BFD session is administratively enabled. Hence, a BFD session over a p2p LSP, using principles of or , can be established faster if the MultipointTail has been provisioned with the value of My Discriminator used by the MultipointHead for that BFD session. That value can be provided to the MultipointTail using different mechanisms, e.g., an extension to IGP. Description of mechanism to provide the value of My Discriminator used by the MultipointHead for the particular BFD session is outside the scope of this specification. Unsolicited notification of the detected failure by the MultipointTail to the MultipointClient performs as described in for the case after the ingress BFD system switches the remote peer into the Demand mode.