The following processing rules apply for SR tunnel programming.
If the prefix SID is resolved from a prefix SID sub-TLV advertised within an IP Reachability 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 IS-IS 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 IS-IS prefix is not SR-enabled (no SR NHLFE) or an import policy rejects the prefix (SR NHLFE is deprogrammed).
The LDP FEC can also be resolved by using the same or a different IGP instance as that of the prefix SID sub-TLV or by 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 ISIS instance.
The LDP FEC can also be resolved by using a static route, a route within an IS-IS instance, or a route within an OSPF instance. The IS-IS or OSPF instances can be the same as, or different form the IGP instance that advertised the mapping server prefix SID sub-TLV.
The SR ILM label is swapped to an SR NHLFE label. This is only possible if a route is exported from another IGP instance into the local IGP instance without propagating the prefix SID sub-TLV with the route. Otherwise, the SR ILM label is swapped to an SR NHLFE label toward the stitching node.