When LDP programs the primary ILM record in the data path, it provides the IOM with the Protect-Group Identifier (PG-ID) associated with this ILM and which identifies which upstream LSR is protected.
For the system to perform a fast switchover to the backup ILM in the fast path, LDP applies to the primary ILM uniform FRR failover procedures similar in concept to the ones applied to an NHLFE in the existing implementation of LDP FRR for unicast FECs. There are however important differences to note. LDP associates a unique Protect Group ID (PG–ID) to all mLDP FECs which have their primary ILM on any LDP interface pointing to the same upstream LSR. This PG-ID is assigned per upstream LSR regardless of the number of LDP interfaces configured to this LSR. Therefore, this PG-ID is different from the one associated with unicast FECs and which is assigned to each downstream LDP interface and next-hop. However, if a failure caused an interface to go down and also caused the LDP session to upstream peer to go down, both PG-IDs have their state updated in the IOM and therefore the uniform FRR procedures are triggered for both the unicast LDP FECs forwarding packets toward the upstream LSR and the mLDP FECs receiving packets from the same upstream LSR.
When the mLDP FEC is programmed in the data path, the primary and backup ILM records therefore contain the PG-ID the FEC is associated with. The IOM also maintains a list of PG-IDs and a state bit which indicates if it is UP or DOWN. When the PG-ID state is UP the primary ILM for each mLDP FEC is open and accepts mLDP packets while the backup ILM is blocked and drops mLDP packets. LDP sends a PG-ID DOWN notification to IOM when it detects that the LDP session to the peer is gone down. This notification causes the backup ILMs associated with this PG-ID to open and accept mLDP packets immediately. When IGP re-converges, an updated pair of primary and backup ILMs is downloaded for each mLDP FEC by LDP into the IOM with the corresponding PG-IDs.
If multiple LDP interfaces exist to the upstream LSR, a failure of one interface brings down the link Hello adjacency on that interface but not the LDP session which is still associated with the remaining link Hello adjacencies. In this case, the upstream LSR updates in IOM the NHLFE for the mLDP FEC to use one of the remaining links. The switchover time in this case is not managed by the uniform failover procedures.