Multi layer path computation with PCE part II

From RFC5623

The calculated path is informed to the invoking LSR as a Explicit Route Object and is used for signaling by the RSVP-TE in a MPLS or GMPLS control plane. Two types of computed path may be possible: 

Otpion 1: Mono-Layer Path: 

• Case 1: the lower layer path included in the end-to-end computed path is already established or will be established on demand. This path specified by a ERO contains TE-links may be a regular TE link that is already established or a virtual TE-link that still needs to be established (RFC5212). In the case of a virtual TE link that must be established a setup attempt for a new lower-layer LSP is initiated once the signaling reaches the head-end of the lower-layer LSP. Since the path of a virtual TE-link is not necessarily know in advance a further lower-layer path computation may be needed. 
•  Case 2: the path computed by the PCE contains a loose hop that spans the lower-layer network, it selects which lower-layer to use but does not selects the path across it. A transit Label Switched Router that is the entry for the lower-layer network is responsible for expanding the loose hop in tis own network. The path expansion on the border LSR may rely on the selection of a existing lower-layer LSP or in the computation and set-up of a new one. 

Option 2: Multi-Layer Path:
The path computed by the PCE is a multi-layer path, it contains TE links from diverse layers (RFC4206). This multi-layer path can contain an already established complete path from the lower layer or if the lower layer path is not already established the signaling of the higher layer LSP triggers the establishment of the lower-layer LSP.

Takeda, T.; Oki, E.; Marzin, A.; Farrel, A. & Roux, J. (2009), 'Framework for PCE-Based Inter-Layer MPLS and GMPLS Traffic Engineering', Framework .