The establishment of TE LSPs and subLSPs is accomplished by PE and P devices through the use of the Resource Reservation Protocol with TE extensions. RSVP-TE is responsible for assigning tunnel labels to a TE LSP or subLSP, managing quality of service issues, and handling error conditions.


A TE LSP may be PE to PE, PE to P, P to PE, or P to P. A subLSP may only be PE to PE.

The headend of a TE tunnel initiates the establishment of an RSVP session and a TE LSP by sending an RSVP Path message toward the tailend of the TE tunnel. When the headend successfully receives from the tailend an RSVP Resv message that matches the sent RSVP Path message, an RSVP session and a TE LSP are established between the headend and the tailend, as shown in RSVP session.

Figure 1. RSVP session

For a P2MP LSP, RSVP Path and Resv messages are used in a similar manner to establish the subLSPs between the PE source (headend) and the two or more PE destinations (tailends).

Peer RSVP-TE speakers establish a TCP- or UDP-based communications session with each other. During the establishment of the session, the tailend allocates a label for the TE LSP or subLSP, places the label in an RSVP Resv message, and sends the Resv message toward the headend. By using the Resv message, the devices in the reverse path exchange tunnel label data to construct the TE LSP or subLSP.

The MPLS Topology Server discovers the RSVP-TE speakers and RSVP sessions that are associated with TE LSPs and subLSPs.