LSP and Tunnel – Same or Different?
Tunnel is a service construct and LSP is a means to realize that service.
-- Tunnel/LSP Identification in RSVP-TE --
An LSP is uniquely identified by a 5-tuple, mentioned below along with their corresponding RSVP objects in which they get carried in PATH/RESV messages.
-SESSION Object PATH/RESV-
- Tunnel End Point Address (Tunnel Destination)
- Tunnel ID (16 bit)
- Extended Tunnel ID (32 bit), generally set to ZERO or the tunnel source
-SENDER_TEMPLATE object [PATH] & FILTER_SPEC object [RESV]-
4. Tunnel Ingress Node (Tunnel Source)
5. LSP ID (16 bit) - The LSP ID is used to differentiate LSPs that belong to the same Tunnel (as identified by its Tunnel ID).
Note: In Ciena SAOS implementation Tunnel Name is also sent in Session Attribute OBJECT of the PATH message as “Session Name”.
So an LSP can be thought of as an instantiation of a tunnel. A single RSVP-TE MPLS Tunnel can have more than one LSPs e.g. primary and backup, or to support make-before-break construct to minimize the traffic loss while increasing BW of a TE tunnel at run time, or re-optimizing the tunnel to take a better path. In that case various instances of that tunnel will have all the values of tuples common other than the LSP id, which will differentiate the two instances belonging to same tunnel.