Also introduced with SR-TE LSP is the indication by the user if the path of the LSP must use protected or unprotected adjacencies exclusively for all links of the path.
When SR OS routers form an IGP adjacency over a link and segment-routing context is enabled in the IGP instance, the static or dynamic label assigned to the adjacency is advertised in the link adjacency SID sub-TLV. By default, an adjacency is always eligible for LFA/RLFA/TI-LFA protection and the B-flag in the sub-TLV is set. The presence of a B-flag does not reflect the instant state of the availability of the adjacency LFA backup; it reflects that the adjacency is eligible for protection. The SR-TE LSP using the adjacency in its path still comes up if the adjacency does not have a backup programmed in the data path at that instant. Use the configure>router>isis>interface> no sid-protection command to disable protection. When protection is disabled, the B-flag is cleared and the adjacency is not eligible for protection by LFA/RLFA/TI-LFA.
SR OS also supports the adjacency set feature that treats a set of adjacencies as a single object and advertises a link adjacency sub-TLV for it with the S-flag (SET flag) set to 1. The adjacency set in the SR OS implementation is always unprotected, even if there is a single member link in it and therefore the B-flag is always clear. Only a parallel adjacency set, meaning that all links terminate on the same downstream router, are used by the local CSPF feature.
Be aware that the same P2P link can participate in a single adjacency and in one or more adjacency sets. Therefore, multiple SIDs can be advertised for the same link.
Third party implementations of Segment Routing may advertise two SIDs for the same adjacency: one protected with B-flag set and one unprotected with B-flag clear. SR OS can achieve the same behavior by adding a link to a single-member adjacency SET, in which case a separate SID is advertised for the SET and the B-flag is cleared while the SID for the regular adjacency over that link has its B-flag set by default. In all cases, SR OS CSPF can use all local and remote SIDs to compute a path for an SR-TE LSP based on the desired local protection property.
There are three different behaviors of CSPF introduced with SR-TE LSP with respect to local protection:
When the local-sr-protection command is not enabled (no local-sr-protection) or is set to preferred, the local CSPF prefers a protected adjacency over an unprotected adjacency whenever both exist for a TE link. This is done on a link-by-link basis after the path is computed based on the LSP path constraints. This means that the protection state of the adjacency is not used as a constraint in the path computation. It is only used to select an SID among multiple SIDs once the path is selected. Thus, the computed path can combine both types of adjacencies.
If a parallel adjacency set exists between two routers in a path and all the member links satisfy the constraints of the path, a single protected adjacency is selected in preference to the parallel adjacency set which is selected in preference to a single unprotected adjacency.
If multiples ECMP paths satisfy the constraints of the LSP path, one path is selected randomly and then the SID selection above applies. There is no check if the selected path has the highest number of protected adjacencies.
When the local-sr-protection command is set to a value of mandatory, CSPF uses it as an additional path constraint and selects protected adjacencies exclusively in computing the path of the SR-TE LSP. Adjacency sets cannot be used because they are always unprotected.
If no path that satisfies the other LSP path constraints and consists of all TE links with protected adjacencies, the path computation returns no path.
Similarly, when the local-sr-protection command to none, CSPF uses it as an additional path constraint and selects unprotected adjacencies exclusively in computing the path of the SR-TE LSP.
If a parallel adjacency set exists between two routers in a path and all the member links satisfy the constraints of the path, it is selected in preference to a single unprotected adjacency.
If no path satisfies the other LSP path constraints and consists of all TE links with unprotected adjacencies, the path computation returns no path.
The local-sr-protection command impacts PCE-computed and PCE-controlled SR-TE LSP. When the local-sr-protection command is set to the default value preferred, or to the explicit value of mandatory, the local-protection-desired flag (L-flag) in the LSPA object in the PCReq (Request) message or in the PCRpt (Report) message is set to a value of 1.
When the local-sr-protection command is set to none, the local-protection-desired flag (L-flag) in the LSPA object is cleared. The PCE path computation checks this flag to decide if protected adjacencies are used in preference to unprotected adjacencies (L-flag set) or must not be used at all (L-flag clear) in the computation of the SR-TE LSP path.