Seamless BFD (S-BFD), RFC 7880, Seamless Bidirectional Forwarding Detection (S-BFD), is a form of BFD that avoids the negotiation and state establishment for the BFD sessions. This is done primarily by pre-determining the session discriminator and then using other mechanisms to distribute the discriminators to a remote network entity. This allows client applications or protocols to more quickly initiate and perform connectivity tests. Furthermore, a per-session state is maintained only at the head end of a session. The tail end simply reflects BFD control packets back to the head end.
A seamless BFD session is established between an initiator and a reflector. There is only one instance of a reflector per SR OS router. A discriminator is assigned to the reflector. Each of the initiators on a router is also assigned a discriminator.
By default, S-BFD operates in asynchronous mode where the reflector encapsulates and routes IP/UDP encapsulated S-BFD packets back to the initiator using the IGP shortest path. However, some applications also support a controlled return TE path for S-BFD reply packets, where S-BFD operates in echo mode and the reflector router forwards packets back toward the initiator on a specified labelled path using, for example, an SR policy. For more information, see the application-specific descriptions for S-BFD in the 7750 SR and 7950 XRS Segment Routing and PCE User Guide.
Seamless BFD sessions are created on the request of a client application, for example, MPLS. This user guide describes the base S-BFD configuration required on initiator and reflector routers. Application-specific configuration is required to create S-BFD sessions.