The following are the MPLS and TE database features for extending CSPF support to SR-TE LSP:
Supports IPv4 SR-TE LSP
Supports local CSPF on both primary and secondary standby paths of an IPv4 SR-TE LSP
Supports local CSPF in LSP templates of types mesh-p2p-srte and one-hop-p2p-srte of SR-TE auto-LSP
Supports path computation in single area OSPFv2 and IS-IS IGP instances
Computes full explicit TE paths using TE links as hops and returning a list of SIDs consisting of adjacency SIDs and parallel adjacency set SIDs. SIDs of a non-parallel adjacency set is not used in CSPF. The details of the CSPF path computation are provided in SR-TE Specific TE-DB Changes. Loose-hop paths, using a combination of node SID and adjacency SID, are not required.
Uses random path selection in the presence of ECMP paths that satisfy the LSP and path constraints. Least-fill path selection is not required.
Provides an option to reduce or compress the label stack such that the adjacency SIDs corresponding to a segment of the explicit path are replaced with a node SID whenever the constraints of the path are met by all the ECMP paths to that node SID. The details of the label reduction are provided in SR-TE LSP Path Label Stack Reduction.
Uses legacy TE link attributes as in RSVP-TE LSP CSPF
Uses timer re-optimization of all paths of the SR-TE LSP that are in the operational UP state. This differs from RSVP-TE LSP resignal timer feature which re-optimizes the active path of the LSP only.
MPLS provides the current path of the SR-TE LSP and TE-DB updates the total IGP or TE metric of the path, checking the validity of the hops and labels as per current TE-DB link information. CSPF then calculates a new path and provides both the new and metric updated current path back to MPLS. MPLS programs the new path only if the total metric of the new computed path is different than the updated metric of the current path, or if one or more hops or labels of the current path are invalid. Otherwise, the current path is considered one of the most optimal ECMP paths and is not updated in the data path.
Timer resignal applies only to the CSPF computation method and not to the ip-to-label computation method.
Uses manual re-optimization of a path of the SR-TE LSP. In this case, the new computed path is always programmed even if the metric or SID list is the same.
Supports ad-hoc re-optimization. This SR-TE LSP feature for SR-TE LSP triggers the ad-hoc resignaling of all SR-TE LSPs if one or more IGP link down events are received in TE-DB.
Once the re-optimization is triggered, the behavior is the same as the timer-based resignal or the delay option of the manual resignal. MPLS forces the expiry of the resignal timer and asks TE-DB to re-evaluate the active paths of all SR-TE LSPs. The re-evaluation consists of updating the total IGP or TE metric of the current path, checking the validity of the hops and labels, and computing a new CSPF for each SR-TE LSP. MPLS programs the new path only if the total metric of the new computed path is different than the updated metric of the current path, or if one or more hops or labels of the current path are invalid. Otherwise, the current path is considered one of the most optimal ECMP paths and is not updated in the data path.
Supports using unnumbered interfaces in the path computation. There is no support for configuring an unnumbered interface as a hop in the path of the LSP is not required. So, the path can be empty or include hops with the address of a system or loopback interface but path computation can return a path that uses TE links corresponding to unnumbered interfaces.
Supports admin-group, hop-count, IGP metric, and TE-metric constraints
Bandwidth constraint is not supported since SR-TE LSP does not have an LSR state to book bandwidth. Thus, the bandwidth parameter, when enabled on the LSP path, has no impact on local CSPF path calculation. However, the bandwidth parameter is passed to PCE when it is the selected path computation method. PCE reserves bandwidth for the SR-TE LSP path accordingly.