Alcatel-Lucent 7450 Manual page 576

Uniform Failover from Primary to Backup ILM
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. As such this PG-ID is different from the one associated with unicast
FECs and which is assigned to each downstream LDP interface and next-hop. If however 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 thus the uniform FRR procedures will be triggered
for both the unicast LDP FECs forwarding packets towards 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 record thus
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 will accept 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 will cause 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.
Note that if multiple LDP interfaces exist to the upstream LSR, a failure of one interface will bring
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.
Page 576 7450 ESS MPLS Guide