PCC-initiated and PCE-computed or controlled LSP

In this mode of operation, the ingress LER uses Path Computation Element Communication Protocol (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).

The following PCE control modes are supported:

Passive Control Mode: In this mode, the user enables the path-computation-method pce command for one or more SR-TE LSPs and a PCE performs path computations at the request of the PCC.
Active Control Mode: In this mode, the user enables the pce-control command for an LSP, which allows the PCE to perform both path computation and periodic reoptimization of the LSP path without an explicit request from the PCC.

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

The following procedure describes configuring and programming a PCC-initiated SR-TE LSP when passive or active control is given to the PCE.

The SR-TE LSP configuration is created on the PE router via CLI or via OSS/SAM. The configuration dictates which PCE control mode is needed: active (pce-control option enabled) or passive (path-computation-method pce enabled and pce-control disabled).
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. 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.
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 PCE, the PCE ignores them and computes the path with the other constraints, excepting the IRO.
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.
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: 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 IGP. This causes 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.
The PE router updates the CPM 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.
PE router sends a PCRpt message to update PCE with the state of up 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.
Following a network event or re-optimization, 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.

Note: If the no pce-control command is performed while a PCUpdate MBB is in progress on the LSP, the router aborts and removes the information and state related to the in-progress PCUpdate MBB. As the LSP is no longer controlled by the PCE, the router may take further actions depending on the state of the LSP. For example, if the LSP is up, and has FRR active or pre-emption, then the router starts a GlobalRevert or pre-emption MBB. If the LSP is down, the router starts the retry-timer to trigger setup.
The PE router updates the CPM and the data path with the new path.
The PE router sends a new PCRpt message to update PCE with the state of up and the RRO as confirmation. The state of the LSP is now synchronized between the router and the PCE.
If the user makes any configuration change to the PCE-computed or PCE-controlled LSP, MPLS requests PCC to first revoke delegation in a PCRpt message (PCE-controlled only), and then MPLS and PCC follow the above steps to convey the changed constraint to PCE, which results in a new path programmed into the data path, the LSP databases of PCC and PCE to be synchronized, and the delegation to be returned to PCE.

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

Note: The preceding procedure is followed when the user performs a no shutdown on a PCE-controlled or PCE-computed LSP. The starting point is an administratively-down LSP with no active paths.

The following steps are followed for an LSP with an active path:

If the user enabled the path-computation-method pce option on a PCC-controlled LSP which has an active path, no action is performed until the next time the router needs a path for the LSP following a network event of 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 which has an active path, PCC issues a PCRpt message to PCE with the state of up and the RRO of the active path. It sets delegation control flag to delegate control to PCE. PCE keeps the active path of the LSP and does not update until the next network event or re-optimization. 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 PCE, the user must configure the PCE path profile ID and path group ID the LSP belongs to. These parameters are passed transparently by PCC to PCE and, so, 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 PCE 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 section of this guide.