Which of the following are characteristics of an LSP? (Choose two)

An MPLS Label Switched Path (LSP) is fundamentally a unidirectional path from an ingress Label Edge Router (LER) to an egress LER. To establish bidirectional communication, two separate unidirectional LSPs are required. Each LSP is created for a specific Forwarding Equivalence Class (FEC), which is a group of packets that are forwarded in the same manner. The LSP acts as a tunnel, ensuring all packets for that FEC traverse the same pre-determined path through the MPLS network.

B. LSPs are inherently unidirectional. Bidirectional services are built using two unidirectional LSPs, not a single bidirectional LSP.
C. A secondary path is an optional feature for LSP protection (fast reroute or secondary standby) and is not a mandatory characteristic of all LSPs.
E. While LDP-signaled LSPs typically follow the IGP best path, RSVP-TE allows for traffic-engineered LSPs that can take explicit paths different from the IGP shortest path.

1. RFC 3031, "Multiprotocol Label Switching Architecture", Section 2.1, "Labels and Labelled Packets". (States that an LSP is a one-way path).

2. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR MPLS Guide, Release 22.10.R1, Chapter: MPLS Overview, Section: Label Switched Paths (LSPs). (Describes LSPs as tunnels for specific FECs).