For PCC-controlled SR-TE LSPs, CSPF is supported on the router using the path-computation-method local-cspf command. See SR-TE LSP path computation using local CSPF for details about the full CSPF path computation method. By default, the path is computed using the hop-to-label translation method. In the latter case, MPLS makes a request to the TE-DB to get the label corresponding to each hop entered by the user in the primary path of the SR-TE LSP. See SR-TE LSP path computation using hop-to-label translation for details of the hop-to-label translation.
The user can configure the path computation request of a CSPF-enabled SR-TE LSP to be forwarded to a PCE instead of the local router CSPF (path-computation-method local-cspf option enabled) by enabling the path-computation-method pce option, as described in SR-TE LSP instantiation. The user can further delegate the re-optimization of the LSP to the PCE by enabling the pce-control option. In both cases, PCE is responsible for determining the label required for each returned explicit hop and includes this in the SR-ERO.
In all cases, the user can configure the maximum number of labels which the ingress LER can push for a specific SR-TE LSP by using the max-sr-labels command.
This command is used to set a limit on the maximum label stack size of the SR-TE LSP primary path so as to allow room to insert additional transport, service, and other labels when packets are forwarded in a context.
config>router>mpls>lsp>max-sr-labels label-stack-size [additional-frr-labels labels]
The max-sr-labels label-stack-size value should be set to account for the needed maximum label stack of the primary path of the SR-TE LSP. Its range is 1-11 and the default value is 6.
The value in additional-frr-labels labels should be set to account for additional labels inserted by remote LFA or Topology Independent LFA (TI-LFA) for the backup next hop of the SR-TE LSP. Its range is 0-3 labels with a default value of 1.
The sum of both label values represents the worst case transport of SR label stack size for this SR-TE LSP and is populated by MPLS in the TTM such that services and shortcut applications can check it to decide if a service can be bound or a route can be resolved to this SR-TE LSP. More details of the label stack size check and requirements in various services and shortcut applications are provided in Service and shortcut application SR-TE label stack check.
The maximum label stack supported by the router is discussed in Data path support and always signaled by PCC in the PCEP Open object as part of the SR-PCE-CAPABILITY TLV. It is referred to as the Maximum Stack Depth (MSD).
In addition, the per-LSP value for the max-sr-labels label-stack-size option, if configured, is signaled by PCC to PCE in the Segment-ID (SID) Depth value in a METRIC object for both a PCE-computed LSP and a PCE-controlled LSP. PCE computes and provides the full explicit path with TE-links specified. If there is no path with the number of hops lower than the MSD value, or the SID Depth value if signaled, a reply with no path is returned to PCC.
For a PCC-controlled LSP, if the label stack returned by the TE-D exceeds the per LSP maximum SR label stack size, the LSP is brought down.