The following processing rules apply for SR tunnel programming.
If the prefix SID is resolved from a prefix SID sub-TLV advertised within an OSPF Extended Prefix TLV, one of the following applies:
The SR ILM label is swapped to an SR NHLFE label as in SR tunnel resolution when the next hop of the OSPF prefix is SR-enabled.
The SR ILM label is stitched to an LDP FEC of the same prefix when either the next hop of the OSPF prefix is not SR enabled (no SR NHLFE) or an import policy rejects the prefix (SR NHLFE deprogrammed).
The LDP FEC can also be resolved using the same or a different IGP instance as that of the prefix SID sub-TLV or using a static route.
If the prefix SID is resolved from a mapping server advertisement, one of the following applies:
The SR ILM label is stitched to an LDP FEC of the same prefix, if one exists. The stitching is performed even if an import policy rejects the prefix in the local OSPF instance.
The LDP FEC can also be resolved using a static route, a route within an IS-IS instance, or a route within an OSPF instance. The latter two can be the same as, or different from the IGP instance that advertised the mapping server prefix SID sub-TLV.
The SR ILM label is swapped to an SR NHLFE label toward the stitching node.