More about RSVP
sessions and RSVP session discovery
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.
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.