PCC-Initiated and PCE-Computed/Controlled LSPs

The following is the procedure for configuring and programming a PCC-initiated SR-TE LSP when control is delegated to the PCE.

  1. The LSP configuration is created on the PE router via CLI or via the OSS/NSP NFM‑P.

    The configuration dictates which PCE control mode is desired: active (pce-control and pce-report options enabled) or passive (path-computation-method pce enabled and pce-control disabled).

  2. 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 must keep track of the association of the PLSP-ID to the Tunnel-ID and Path-ID, and use the latter to communicate with MPLS about a specific path of the LSP. PCC also uses the SRP-ID to correlate PCRpt messages for each new path of the LSP.
  3. The PE router does not validate the entered path.
    Note however that in the SR OS, the PCE supports the computation of a path for an LSP with empty-hops in its path definition. While PCC will include the IRO objects in the PCReq message to PCE, the PCE will ignore them and compute the path with the other constraints except the IRO.
  4. The PE router sends a PCReq message to the PCE to request a path for the LSP.
    The PCReq message includes the LSP parameters in the METRIC object, the LSPA object, and the BANDWIDTH object. The PE router also includes the LSP object with the assigned PLSP-ID. At this point, the PCC does not delegate the control of the LSP to the PCE.
  5. The PCE computes a new path, reserves the bandwidth, and returns the path in a 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 any computed metric value, and the BANDWIDTH object.
    Note: For the PCE to be able 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 make sure 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 PCE to learn it in its TE database.
  6. The PE router updates the CPM and the data path with the new path.
    Up to this point, the PCC and PCE are using passive stateful PCE procedures. The next steps will synchronize the LSP database of the PCC and PCE for both PCE-computed and PCE-controlled LSPs. They will 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 an UP state, and also sends the RRO as confirmation.
    It now includes the LSP object with the unique PLSP-ID. For a PCE-controlled LSP, the PE router also sets the 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 a re-optimization, the PCE computes a new path for a PCE-controlled LSP and returns it in a PCUpd message with the new ERO.
    It will include the LSP object with the same unique PLSP-ID assigned by the PCC, as well as 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 CPM and the data path with the new path.
  10. The PE router sends a PCRpt message to inform the PCE that the older path is deleted.
    It includes the unique PLSP-ID value in the LSP object and the R (Remove) bit set.
  11. The PE router sends a new PCRpt message to update PCE with an UP state, and also sends the RRO to confirm the new path.
    The state of the LSP is now synchronized between the router and the PCE.
  12. If PCE owns the delegation of the LSP and is making a path update, MPLS will initiate the LSP and update the operational value of the changed parameters while the configured administrative values will not change.
    Both the administrative and operational values are shown in the details of the LSP path in MPLS.
  13. If the user makes any configuration change to the PCE-computed or PCE-controlled LSP, MPLS requests that the PCC 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 will result in the programming of a new path into the data path, the synchronization of the PCC and PCE LSP databases, and the return of delegation to PCE.

The above procedure is followed when the user performs a no shutdown command on a PCE-controlled or PCE-computed LSP. The starting point is an LSP which is administratively down with no active path. For an LSP with an active path, the following items can apply:

  1. If the user enabled the path-computation-method pce option on a PCC-controlled LSP with an active path, no action is performed until the next time the router needs a path for the LSP following a network event of a LSP parameter change. At that point, the prior procedure is followed.

  2. If the user enabled the pce-control option on a PCC-controlled or PCE-computed LSP with an active path, the PCC will issue a PCRpt message to the PCE with an UP state, as well as the RRO of the active path. It will set the delegation control flag to delegate control to the PCE. The PCE will keep the active path of the LSP and make no updates to it until the next network event or re-optimization. At that point, the prior procedure is followed.

Parent topic: LSP Initiation