PCC-initiated and PCE-computed/controlled LSP

In the PCC-initiated and PCE-computed/controlled LSP mode of operation, the ingress LER uses PCEP to communicate with a PCE-based external TE controller (also referred to as the PCE). The router instantiates a PCEP session to the PCE. The router is referred to as the PCE client (PCC).

When the user enables the pce-computation option for one or more SR-TE LSPs, the PCE performs path computations at the request of the PCC, which is referred to as passive stateful mode. If the user enables the pce-control option for an LSP, the PCE can also perform both path computation and periodic reoptimization of the LSP path without an explicit request from the PCC. This is referred to as active stateful mode.

For the PCC to communicate with a PCE about the management of the path of an SR-TE LSP, the router implements the extensions to PCEP in support of segment routing (see PCEP for more information).This feature works with the Nokia stateful PCE, which is part of the network services platform (NSP).

The following steps describe configuring a PCC-initiated SR-TE LSP when passive or active control is given to the PCE.

1. The SR-TE LSP configuration is created on the PE router using the CLI or NSP NFM-P.

   The configuration dictates which PCE stateful mode is desired: active (pce-control option enabled) or passive (pce-computation enabled and pce-control disabled).

2. The PCC assigns a unique PLSP-ID to the LSP. The PLSP-ID uniquely identifies the LSP on a PCEP session and must remain constant during its lifetime. The PCC on the router tracks the association of {PLSP-ID, SRP-ID} to {tunnel-ID, path-ID} and uses the latter to communicate with MPLS about a specific path of the LSP.

3. The PE router does not validate the entered path. While the PCC can include the IRO objects for any loose or strict hop in the configured LSP path in the path computation request (PCReq) message to the PCE, the PCE ignores the IRO objects and computes the path with the other constraints.

4. The PE router sends a PCReq message to the PCE to request a path for the LSP and includes the LSP parameters in the METRIC object, the LSPA object, and the BANDWIDTH object. It also includes the LSP object with the assigned PLSP-ID. At this point, the PCC does not delegate control of the LSP to the PCE.

5. The PCE computes a new path, reserves the bandwidth, and returns the path in a path computation reply (PCRep) message with the computed ERO in the ERO object. It also includes the LSP object with the unique PLSP-ID, the METRIC object with the computed metric value if any, and the BANDWIDTH object.

   Note: In order for the PCE to use the SRLG path diversity and admin-group constraints in the path computation, the user must configure the SRLG and admin-group membership against the MPLS interface and verify that the traffic-engineering option is enabled in the IGP. This causes the IGP to flood the link SRLG and admin-group membership in its participating area and for the PCE to learn it in its TE database.

6. The PE router updates the CSM and the data path with the new path.

   Up to this step, the PCC and PCE are using passive stateful PCE procedures. The next steps synchronize the LSP database of the PCC and PCE for both PCE-computed and PCE-controlled LSPs. They also initiate the active PCE stateful procedures for the PCE-controlled LSP only.

7. The PE router sends a PCRpt message to update the PCE with the Up state and the RRO as confirmation, including the LSP object with the unique PLSP-ID. For a PCE-controlled LSP, the PE router also sets a delegation control flag to delegate control to the PCE. The state of the LSP is now synchronized between the router and the PCE.

8. Following a network event or reoptimization, the PCE computes a new path for a PCE-controlled LSP and returns it in a path computation update (PCUpd) message with the new ERO. It includes the LSP object with the same unique PLSP-ID assigned by the PCC and the stateful request parameter (SRP) object with a unique SRP-ID number to track error and state messages specific to this new path.

9. The PE router updates the CSM and the data path with the new path.

10. The PE router sends a new PCRpt message to update the PCE with the Up state and the RRO as confirmation. The state of the LSP is now synchronized between the router and the PCE.

11. If the user makes any configuration change to the PCE-computed or PCE-controlled LSP, MPLS requests the PCC to revoke delegation in a PCRpt message (PCE-controlled only), and then MPLS and the PCC follow the above steps to convey the changed constraint to the PCE, which will result in a new path programmed into the data path, the LSP databases of the PCC and PCE to be synchronized, and the delegation to be returned to the PCE.

    For SR-TE LSPs, MBB is not supported. Therefore, the PCC first tears down the LSP and sends a PCRpt message to the PCE with the remove flag set to 1 before following this configuration change procedure.

The following steps are for an LSP with an active path.

• If the user enabled the pce-computation option on a PCC-controlled LSP that has an active path, no action is performed until the next time the router needs a path for the LSP following a network event or an LSP parameter change. At that point, the procedures above are followed.

• If the user enabled the pce-control option on a PCC-controlled or PCE-computed LSP that has an active path, the PCC will issue a PCRpt message to the PCE with the Up state and the RRO of the active path. The PCC will set the delegation control flag to delegate control to the PCE. The PCE will keep the active path of the LSP and will not update until the next network event or reoptimization. At that point, the procedures above are followed.

The PCE supports the computation of disjoint paths for two different LSPs originating or terminating on the same or different PE routers. To indicate this constraint to the PCE, the user must configure the PCE path profile ID and path group ID that the LSP belongs to. These parameters are passed transparently by the PCC to the PCE and are therefore opaque data to the router. The user can configure the path profile and path group using the path-profile profile-id [path-group group-id] command.

The association of the optional path group ID is to allow the PCE to determine which profile ID this path group ID must be used with. One path group ID is allowed per profile ID. The user can, however, enter the same path group ID with multiple profile IDs by executing this command multiple times. A maximum of five entries of path-profile [path-group] can be associated with the same LSP. More details of the operation of the PCE path profile are provided in the PCEP chapter.