When an SR-TE LSP is configured on the router, its path can be computed by the router or by an external TE controller referred to as a Path Computation Element (PCE). This feature works with the Nokia stateful PCE which is part of the Network Services Platform (NSP).The SR OS supports the following modes of operations which are configurable on a per SR-TE LSP basis:
When the path of the LSP is computed by the router acting as a PCE Client (PCC), the LSP is referred to as PCC-initiated and PCC-controlled.
A PCC-initiated and controlled SR-TE LSP has the following characteristics:
Can contain strict or loose hops, or a combination of both.
Supports both a basic hop-to-label translation and a full CSPF as a path computation method.
The capability exists to report an SR-TE LSP to synchronize the LSP database of a stateful PCE server using the pce-report option, but the LSP path cannot be updated by the PCE. In other words, the control of the LSP is maintained by the PCC
When the path of the LSP is computed by the PCE at the request of the PCC, it is referred to as PCC-initiated and PCE-computed.
A PCC-initiated and PCE-computed SR-TE LSP supports the Passive Stateful Mode, which enables the path-computation-method pce option for the SR-TE LSP so PCE can perform path computation at the request of the PCC only. The PCC retains control.
The capability exists to report an SR-TE LSP to synchronize the LSP database of a stateful PCE server using the pce-report option.
When the path of the LSP is computed and updated by the PCE following a delegation from the PCC, it is referred to as PCC-initiated and PCE-controlled.
A PCC-initiated and PCE-controlled SR-TE LSP allows Active Stateful Mode, which enables the pce-control option for the SR-TE LSP so PCE can perform path computation and updates following a network event without the explicit request from the PCC. The PCC delegates full control.
The user can configure the path computation requests only (PCE-computed) or both path computation requests and path updates (PCE-controlled) to PCE for a specific LSP using the path-computation-method pce and pce-control commands.
The path-computation-method pce option sends the path computation request to the PCE instead of the local CSPF. When this option is enabled, the PCE acts in Passive Stateful mode for this LSP. In other words, the PCE can perform path computations for the LSP only at the request of the router. This is used in cases where the operator wants to use the PCE specific path computation algorithm instead of the local router CSPF algorithm.
The default value is no path-computation-method.
The user can also enable the router's full CSPF path computation method. See SR-TE LSP path computation using local CSPF for more details.
The pce-control option allows the router to delegate full control of the LSP to the PCE (PCE-controlled). Enabling it means the PCE is acting in Active Stateful mode for this LSP and allows PCE to reroute the path following a failure or to re-optimize the path and update the router without requiring the router to request it.
The user can delegate LSPs computed by either the local CSPF or the hop-to-label translation path computation methods.
The user can delegate LSPs which have the path-computation-method pce option enabled or disabled. The LSP maintains its latest active path computed by PCE or the router at the time it was delegated. The PCE only makes an update to the path at the next network event or re-optimization. The default value is no pce-control.
PCE report is supported for SR-TE LSPs with more than one path. However, PCE computation and PCE control are not supported in such cases. PCE computation and PCE control are supported for SR-TE LSPs with only one path that is either primary or secondary.
In all cases, the PCC LSP database is synchronized with the PCE LSP database using the PCEP Path Computation State Report (PCRpt) message for LSPs that have the pce-report command enabled.
The global MPLS level pce-report command can be used to enable or disable PCE reporting for all SR-TE LSPs for the purpose of LSP database synchronization. This configuration is inherited by all LSPs of a specified type. The PCC reports both CSPF and non-CSPF LSP. The default value is disabled (no pce-report). This default value controls the introduction of PCE into an existing network and allows the operator to decide if all LSP types need to be reported.
The LSP level pce-report command overrides the global configuration for reporting LSP to PCE. The default value is to inherit the global MPLS level value. The inherit value returns the LSP to inherit the global configuration for that LSP type.
For more information about configuring PCC-Initiated and PCC-Controlled LSPs, see Configuring PCC-controlled, PCE-computed, and PCE-controlled SR-TE LSPs.