Which of the following reservation styles can be used to prevent both the primary and secondary paths from booking resources on a common link?

The Shared Explicit (SE) reservation style is designed for this purpose. It allows multiple LSPs (in this case, a primary and a secondary path) to share a single resource reservation on a common link. The reservation is made for the maximum bandwidth required by any single LSP in the shared set. This ensures that bandwidth is reserved for failover but prevents the primary and standby paths from consuming double the bandwidth on any shared links, as only one path can be active at a time using that shared reservation.

A. The Fixed Filter (FF) style creates a distinct, separate reservation for each LSP, which would lead to double-booking of resources on a common link.
C. A standby explicit LSP is a type of path, not a reservation style. It typically uses the Shared Explicit style to achieve its goal.
D. Preemptive filter is not a standard RSVP reservation style. Preemption is a separate attribute related to LSP priority.
E. Link capacity filter is not a standard RSVP reservation style.

1. Braden, R., Ed., et al., Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification, RFC 2205, IETF, September 1997, Section 3.3, "Reservation Styles". (Defines the Shared-Explicit (SE) style).

2. Awduche, D., et al., RSVP-TE: Extensions to RSVP for LSP Tunnels, RFC 3209, IETF, December 2001, Section 4.4, "Reservation Styles". (Notes that the SE style is useful for tunnels in a hot-standby state).

3. Nokia, SR OS MPLS Guide, Release 21.10.R1, Document ID: 3HE 17179 AAAA TQZZA, 2021, Section: "Reservation Styles", p. 215. (Describes how SE style allows multiple senders to share a flow).