LDP to SR stitching

The responsibility for stitching the labels between the two MPLS domains falls on the border routers that are running both protocols – namely, P3 and P7 in this case.

For the scenario examined currently, the source router is PE5 in the LDP domain and the destination router is PE1 in the SR-MPLS domain.

As soon as a route is injected into the RIB, the LDP allocates a local label for it and then distributes it to its downstream neighbors. Routers P3, P7, P4, P8, and PE5 perform this function even for routers PE1, P2, and P6, as all routers are within the same IS-IS routing domain.

This creates an LDP LSP from the router PE5 to P3 for the destination PE1.

At P3, upon realizing that no outgoing LDP label has been received from P2 for the destination PE1, the entry briefly remains unlabeled. However, P3 identifies that there is an SR-MPLS prefix-SID label from itself to PE1. Prefix then copies this SR-MPLS label over the previously unlabeled entry...